Build Path B — Parts Kit + US-Made Semi-Auto Receiver

The iconic US 'buy a kit and build' SMG project; 922(r) parts-count compliance

6.1 Contents

(Generated by build/inject_toc.py at build time. Section headers below are the source of truth.)

Correction, 2026-10-18. The 2026-05-26 draft of this dive cited a “Bear Arms” STEN 922(r) compliance sheet, Bear Arms drawings, and US-made Sterling-pattern magazines and parts from Bear Arms, ASP and GunSpring. None of these could be found: bear-arms.com is a used-gun dealer whose archived pages from 2011 to 2024 show no STEN material. All of it has been removed across the dive, and the seller and price information now comes from the sellers’ own pages, dated. The same review found that the extractor, selector and magazine catch had been counted as § 922(r) listed parts; they are not.

This is the iconic US “buy a kit and build” SMG project, and for most builders, it’s the right answer. Path A (NFA Class III pre-86 transferable, Vol 5) is the most-authentic path but costs $10–25K and results in a federally-registered machine gun. Path C (from-scratch lab build, Vol 7) is the most-satisfying path for a builder with the lab and the time, but it’s a 40–80-hour project that asks for sheet-metal forming, TIG welding, lathe work, and a 9×19 chamber reamer with headspace gauges. Path B sits in the middle: buy a demilled WWII parts kit for $200–500 (Apex Gun Parts, Sarco, Indianapolis Ordnance, IO Inc.), buy or build a US-made semi-auto tube receiver, do the 922(r) parts-count compliance arithmetic, rework the FCG to closed-bolt-semi-auto (mandatory: ATF Rul. 83-5 classifies an open-bolt semi-auto STEN as a machine gun), and assemble. Two weekends of shop time, $400–1,200 total, and the result is a real STEN that has actually been to the European theater, refinished on the builder’s bench.

Figure 1 — A completed parts-kit Sten build with original WWII components reassembled on a home-shop receiver. Source: gunsamerica.com.

Vol 6 is the canonical Path B treatment. It’s also the place in this hub where the 922(r) parts-count compliance knowledge lives — the same arithmetic applies (with different parts lists) to AK-47 parts-kit builds, Sterling Mk 4 / L2A3 parts-kit builds, and any other imported-SMG-or-rifle parts-kit build under US federal law. Vol 10 owns the rule; this volume owns the application to the STEN. Reading § 6.5 once is sufficient to have the 922(r) framework cold for the rest of the hub.

Two important caveats up front:

Closed-bolt FCG is mandatory for a new semi-auto STEN build. The WWII open-bolt fixed-firing-pin design (Vol 4 § 4.4.2) is the design ATF Rul. 83-5 classifies as a machine gun: an open-bolt, fixed-firing-pin semi-auto STEN with only the trip lever altered. A Path B build cannot use the original FCG as-is in semi-auto; the FCG must be reworked to closed-bolt operation, adding a separate firing pin to the bolt and a firing-pin striker mechanism to the FCG. This is the largest single mechanical departure from a WWII STEN that a parts-kit build requires. Vol 7 § 7.5 covers the closed-bolt FCG fabrication in detail; this volume cites that work rather than reproducing it. 83-5 contains no grandfather date, so no open-bolt semi-auto STEN is exempt (Vol 10 § 10.4; Correction, 2026-10-20: earlier versions cited an “ATF Ruling 80-21” closed-bolt rule with a pre-1982 grandfather, which does not exist).
922(r) parts-count compliance is non-optional. A parts-kit build using an imported demilled kit on a US-made semi-auto receiver must satisfy the 922(r) parts-count rule: no more than 10 of the listed 20 imported parts may remain in the completed firearm. The kit arrives with most of those parts imported; the build path replaces enough of them with US-made (or US-fabricated) parts to land on the legal side of the count. Get the arithmetic wrong and the assembly itself is unlawful (18 USC § 922(r) + 27 CFR § 478.39 prohibit assembling the rifle, not possessing it). § 6.5.2 has the worked count; write your own count down part by part.

6.2 Read-this-first — scope, prerequisites, expected outcome

6.2.1 Scope of this volume

Vol 6 documents a Mk II parts-kit build on a US-made semi-auto tube receiver, closed-bolt semi-auto-only FCG rework, 922(r) compliant, 9×19 chamber, original WWII parts where they don’t conflict with 922(r) and modern aftermarket where they do. This is the canonical Vol 6 target: a kit-built STEN that looks like a Mk II from any reasonable distance, fires semi-auto only, satisfies 922(r), and uses a federally non-NFA receiver.

§ 6.11 covers two stretch options: a Mk V parts-kit build (using a Mk V wood-furniture kit instead of Mk II) and an NFA Class III pre-86 registered transferable receiver option (Path A combined with Path B kit parts; § 6.11.2). The NFA stretch turns the build into a registered machine gun — the same legal regime as Vol 5 (Path A) — and exempts it from the closed-bolt rework requirement.

6.2.2 Prerequisites

Bench gunsmithing competence. A parts-kit build is fundamentally a bench-gunsmithing project, not a machinist project. The builder needs to be comfortable fitting parts, dressing welds, pinning components, filing to size, and verifying headspace. The kit’s barrel and FCG mostly drop in; the receiver-to-kit interface and the closed-bolt FCG rework are the work.
Modest machining capability. Not lab-scale Path C capability — but: a drill press with depth-stop, a hand-grinder or Dremel for dressing, a Dremel-class cutoff tool or hacksaw for the demilled receiver remnant, and ideally a small lathe or mill for the closed-bolt FCG rework (cross-ref Vol 7 § 7.5 — the Professor Parabellum Vol III closed-bolt FCG layout). A builder with no machining can buy the Indianapolis Ordnance semi-auto bolt kit (§ 6.6.2; confirm its contents); a builder with a full lab (Vol 4 § 4.0 lab-capability framing) can fabricate it.
Headspace gauges. A GO + NO-GO 9×19 headspace gauge set is non-negotiable for verifying the kit barrel against the new receiver chamber boss. The kit barrel is original WWII (or refresh-of-WWII production); headspace verification ensures it’s safe to fire in the new receiver.
Lab tooling for receiver work. Drill press, hand files, fine-tooth hacksaw, a small benchtop arbor press or hydraulic press for pinning. Optional but useful: a small belt sander or disc sander for dressing the receiver-to-kit interface. The full Vol 7 sheet-metal-forming + TIG-welding kit is not required for Path B — the build starts with a pre-built (or 80%-finished) US-made receiver.
Legal posture verified. State of residence permits possession + manufacture of a non-NFA semi-auto SMG-pattern carbine for personal use (most states permit; Vol 10 has the per-state map). State of residence SBR rules understood — the kit barrel at ~7.75″ (Mk II) or ~7.8″ (Mk V) is a federal SBR by definition for a US semi-auto build unless extended to ≥16″. Most Path B builders accept SBR posture (Form 1, $200 tax stamp, ~6–10 month wait); some extend the barrel to ≥16″ for non-SBR posture; the trade-off is documented in Vol 10 § 10.5.

6.2.3 Expected outcome

A functional Mk II-pattern closed-bolt semi-auto 9×19 STEN carbine built on a US-made tube receiver, 922(r) compliant, with the visual character of a 1944-production Mk II from Fazakerley or RSAF Enfield (the kit’s source). Weight ~3.2 kg empty, overall length ~30″ with stock attached, fires single-action closed-bolt semi-auto from Sterling-pattern aftermarket magazines. Per-build cost (parts kit + US receiver + 922(r)-compliant aftermarket parts + closed-bolt FCG rework + finishing supplies + SBR tax stamp): $400–1,200 depending on parts-kit pricing volatility, receiver choice (pre-built ~$300 vs 80% ~$150 vs Vol 7 scratch ~$50 materials), and how much of the closed-bolt FCG is fabricated vs purchased.

Time-on-task: 15–30 lab hours for a first build, distributed across the receiver-to-kit fit-up, closed-bolt FCG rework, headspace verification, and finishing. About two weekends of focused bench time. This is significantly less than Vol 7’s 40–80 hours because the major fabrication work (receiver tube formation, barrel rifling, magazine spring fabrication) is bypassed — the kit and the US-made receiver carry most of the load.

6.3 The US parts-kit market — vendors, pricing, what arrives in the box

The STEN is one of the most-abundant parts-kit guns in the US surplus market. Approximately 4 million STENs were produced 1941–1953 (Vol 2 § 2.4), and the post-war demilitarization of UK, Canadian, Commonwealth, and various European surplus stocks fed a steady stream of demilled kits into the US market from the 1960s onward. Apex Gun Parts has been the most-consistent US vendor; Sarco, IO Inc., and Indianapolis Ordnance have all carried STEN kits in rotation; smaller dealers (Numrich Gun Parts, gun-show vendors, estate sales) carry kits intermittently.

6.3.1 Primary US vendors

The US parts-kit landscape, checked on the sellers’ own pages on 2026-10-18:

Table 1 — The US parts-kit landscape, checked on the sellers' own pages on 2026-10-18

Vendor	STEN stock seen 2026-10-18	Price	Notes
Apex Gun Parts (apexgunparts.com)	Mk 2 parts kit with Mk 5 barrel nut, no stock; two Mk III kits; individual parts, surplus magazines, “T” stock, a new US-made Mk 2 barrel	Mk 2 kit $279; Mk III $124.95 and $249.95; barrel $325	The largest STEN listing found. No 922(r) documentation was seen on its pages.
Indianapolis Ordnance (indianapolisordnance.com)	Semi-auto (SA) bolt kits, an SA tube, barrels, small parts	SA bolt kit $280; SA tube $80 (out of stock)	Sells the semi-auto conversion parts rather than whole kits; a “new US made” disconnector is $15.
Sarco, IO Inc., Numrich	Not checked for this volume	—	Named by builders as STEN sources; verify current stock before relying on them.
gun-show vendors / estate sales	Variable	Variable	Verify completeness against the parts list in § 6.3.1 before buying.

Pricing has trended upward since the early 2000s (when complete Mk II kits were sometimes available for $80–150) and is now sensitive to the broader collector-firearm market — high-condition Mk V kits with intact wood furniture and No-4 sights routinely exceed $500 at gun shows.

6.3.2 What a parts kit actually contains

A “complete” STEN parts kit is everything except the receiver tube (the receiver tube being the federally-defined “firearm” under 18 USC § 921(a)(3) — what BATFE counts as the gun). The kit arrives with:

The demilled receiver remnant — the original WWII receiver tube cut into sections per BATFE demilling regulations (typically three torch-cut sections; the cuts must render the receiver unusable as a firearm without significant refabrication work). The remnant is typically not useful for building the new receiver but is sometimes used as a source of cocking-handle-slot dimensions and ejection-port placement.
The bolt assembly (bolt, fixed firing pin, extractor, cocking-handle pin). Original WWII production; condition variable.
The trigger group housing + FCG (trigger, sear, disconnector, springs, magazine catch, fire-mode selector if Mk II UK; cast aluminum if Long Branch). Original WWII production.
The barrel with chamber and rifling. Original WWII production, 9×19, ~197 mm (Mk II) or ~196 mm (Mk V). Bore condition variable — many kits have surplus barrels with corrosive-primer pitting; some kits have post-war refresh barrels in better condition.
The recoil spring.
The stock — skeleton T-frame, wire-loop, or wood depending on Mk and Mk variant.
Sights — fixed Mk II sights, or Lee-Enfield No-4 sight assembly for Mk V.
One or two magazines (sometimes; not always included). Original WWII Sten magazines have known feed-lip pathology (Vol 4 § 4.7.2 + Vol 11 § 11.3) — buy several, gauge and true the feed lips, and keep the best (Vol 11 § 11.3.2); no newly made US STEN magazine was found (2026-10-18).
Original parkerizing or sun-corite paint on metal parts. Often weathered; some builders refinish, some preserve.
Cocking handle, muzzle bushing, rear cap. Original WWII production.
Sometimes: a buttplate, a foregrip (Mk V), a bayonet lug (Mk V), miscellaneous small parts.

A “partial” kit is some subset of the above — often missing the stock, magazines, or sights. Partial kits are useful for an existing-build replacement-parts donor or for combining with parts from a second source; for a new build, hold out for a complete kit.

6.3.3 Condition expectations and what to inspect at purchase

The kits are 80+ years old. Realistic condition expectations:

Bolt: usually serviceable. Inspect the fixed firing-pin tip for impact damage (a chipped or mushroomed tip needs work or a new bolt); inspect the bolt OD for excessive wear (the WWII surplus bolts are typically still within +0.2 mm of the original 32 mm OD spec).
Barrel: variable. Inspect the bore with a bore mirror or borescope. Light corrosion pitting is common and tolerable for a range-toy build; heavy pitting (visible craters in the rifling) compromises accuracy and gas seal. A barrel with a smooth bore is uncommon but not rare. Inspect the chamber for cracks — kits that were demilled by torch-cutting near the chamber sometimes have heat-affected zones at the chamber that can crack under firing. Reject any kit with a visibly cracked chamber boss.
FCG housing: usually serviceable. The stamped sheet UK Mk II housings are dimensionally robust; the cast aluminum Long Branch housings sometimes have hairline cracks at the magazine catch — inspect with a magnifier.
Springs: original springs are often weak. Plan to replace all springs with modern aftermarket (music wire ASTM A228) springs sized per Vol 4 § 4.5.4.
Stock: usually intact for the Mk II skeleton T-frame (it’s solid steel rod welded in a T); Mk V wood stocks are more variable — check for cracks at the wrist and at the buttplate-attachment screws.
Sights: usually intact for the fixed Mk II flip-aperture rear; Mk V Lee-Enfield No-4 sights are sometimes loose or with damaged adjustment threads — inspect carefully if the kit is sold as a Mk V.
Demilled receiver remnant: irrelevant to the build (not used as a receiver). Useful as a dimensional reference for slot/port placement on the new receiver.

A reputable vendor (Apex, Sarco, IO Inc.) typically grades the kits and provides representative photos. Gun-show purchases are caveat emptor — bring a bore light, a magnifier, calipers, and a copy of the § 6.3 parts list for the inspection.

6.4 Anatomy of a Sten parts kit — the parts-tree and what’s imported

6.4.1 The parts list (Mk II canonical)

A complete Mk II kit, organized by sub-assembly, with each part’s typical origin (imported = original WWII production, captured by 922(r) as an imported part; US = US-made aftermarket or US-fabricated). The 922(r) counted parts (the 20-parts list) are flagged with [922(r)] — see § 6.5 for the count arithmetic.

Receiver assembly (the federally-defined firearm):

Receiver tube — NOT IN THE KIT (kit has only the demilled remnant; the builder supplies the US-made replacement receiver). [922(r) #1: receiver]

Bolt assembly:

Bolt body — imported. [922(r) #4: bolt]
Fixed firing pin (integral to bolt on WWII Mk II) — imported (part of bolt) or US-fabricated as part of closed-bolt FCG rework.
Extractor — imported. Not a 27 CFR 478.39(c) listed part.
Cocking-handle pin — imported.
Cocking-handle knob — imported.

Trigger group housing + FCG:

Trigger group housing — imported. [922(r) #6: trigger housing]
Trigger — imported. [922(r) #7: trigger]
Sear — imported. [922(r) #8: sear (hammer)]
Disconnector — imported. [922(r) #9: disconnector]
Fire-mode selector (Mk II UK) — imported. Not a listed part; it is removed in a semi-auto build in any case.
Magazine catch — imported. Not a listed part (#18 is magazine bodies).
FCG springs — imported (replace with US music wire — see § 6.5.3).
FCG internal pins — imported (replace with US music-wire pins — see § 6.5.3).

Barrel assembly:

Barrel — imported. [922(r) #2: barrel]
Muzzle bushing — imported.

Operating-mechanism parts:

Recoil spring — imported.
Recoil-spring guide — imported.

Stock:

Skeleton T-frame stock (Mk II) — imported. [922(r) #5: buttstock]

Magazine (typically one included, sometimes two):

Magazine body — imported. [922(r) #15: magazine body]
Magazine follower — imported. [922(r) #16: magazine follower]
Magazine floorplate — imported. [922(r) #17: magazine floorplate]

Miscellaneous:

Rear cap — imported.
Front sight blade — imported.
Rear sight assembly (Mk II flip aperture) — imported.

The full 20-parts 922(r) list (per 27 CFR § 478.39) is enumerated in § 6.5.1. Not every part above corresponds to a 922(r)-counted part — the rule lists 20 specific parts, and only some of the parts in a Sten kit map to those 20 categories. The mapping is documented in § 6.5.

6.4.2 What’s missing from the kit (what the builder must supply)

Receiver tube — the federally-defined firearm. The builder supplies: a US-made pre-built semi-auto tube receiver, or an 80% receiver blank to finish, or a from-scratch receiver per Vol 7 § 7.4.
922(r)-compliance US-made replacement parts — typically the buttstock, magazine body + follower + floorplate, sear, disconnector, and trigger. The exact list depends on the final count (§ 6.5).
Closed-bolt FCG components — the firing-pin striker, the separate firing pin in the bolt, the firing-pin return spring, the modified sear engagement geometry. The Professor Parabellum Vol III drawings (Vol 4 § 4.1.3) provide the layout; Indianapolis Ordnance sells a STEN Mk 2 semi-auto bolt kit ($280, 2026-10-18) as the commercial alternative.
Modern Sterling-pattern magazines — at least two, preferably four. WWII original Sten magazines have feed-lip pathology (Vol 11 § 11.3); modern aftermarket are uniformly better.
Headspace gauges (GO + NO-GO 9×19) — non-negotiable. Brownells or PTG, ~$40 for the pair.
Finishing supplies — if refinishing, parkerizing solution and tank (or hot-blue salts, or Cerakote spray rig).
Form 1 SBR paperwork (if the barrel stays at ~7.75″ and the build is configured as an SBR) — $200 tax stamp + 6–10 month wait; Vol 10 § 10.5 has the procedure.

6.5 US-made semi-auto receivers — three options

The kit provides everything except the receiver. The receiver is the federally-defined firearm. Three options exist for sourcing the US-made semi-auto receiver, in increasing order of effort and decreasing order of cost:

6.5.1 Option 1 — pre-built US-made semi-auto receiver (drop-in)

Cost: about $80 when in stock. Effort: minimal for the tube itself; the closed-bolt parts are separate.

A pre-formed semi-auto tube saves the forming and welding of § 6.4.3. What was actually on sale on 2026-10-18:

Indianapolis Ordnance — “STEN MK2 SA Tube”, $80, out of stock. Its semi-auto bolt kits ($280) were in stock. (The Sterling Mk 4 SA tube, $100, was also out of stock.)
Wise Lite Arms — formerly a semi-auto Sterling and STEN maker in Texas; its domain (wiselitearms.com, redirecting to wlarms.com) no longer resolves (2026-10-18). Current status unknown.
An earlier draft of this volume listed other receiver makers and prices; none could be confirmed on a maker’s page, and they have been removed.

With no tube in stock, Options 2 and 3 below, or the lab-made receiver of Vol 7 § 7.4, are the realistic routes.

Pros: minimal effort, professionally formed and welded (no risk of receiver-tube failure from amateur welding), known-good dimensions, often includes the closed-bolt FCG features.

Cons: cost; less control over receiver-tube dimensions and finish; reliance on the manufacturer’s compliance posture (verify the receiver is sold as a non-NFA semi-auto-only firearm under federal law).

6.5.2 Option 2 — 80% receiver blank (finish-in-lab)

Cost: $100–180. Effort: 4–8 hours of finishing work.

An “80% receiver” is a partially-formed receiver tube that, under BATFE precedent, has not yet been “manufactured” as a firearm — it requires final machining operations (typically the magazine-well slot, ejection port, and FCG attachment holes) to become a firearm. Under federal law, an individual may manufacture an 80% receiver into a complete receiver for personal use without serialization (the manufacture-for-personal-use exemption — Vol 10 § 10.6). Several states (CA, NY, NJ, others) require 80% receivers be shipped via FFL and impose serialization requirements; the state-by-state rules are in Vol 10 § 10.5.

The 80% blank arrives as a rolled and welded tube with most of the external dimensions complete; the final operations are the magazine-well slot (rectangular cutout, 60 mm × 30 mm), the ejection port (55 mm × 24 mm), and the trigger-group attachment holes. CNC milling on a benchtop machine handles these operations comfortably (cross-ref Vol 7 § 7.4 Step A8 for the operations).

Pros: significant cost savings vs pre-built; control over final-machining quality; no reliance on a manufacturer’s compliance posture.

Cons: requires CNC mill or drill press + hand tools for the final-machining operations; per-state legal complexity for the 80% blank purchase; documentation burden for the manufacture-for-personal-use exemption.

6.5.3 Option 3 — full from-scratch receiver (Vol 7 § 7.4)

Cost: $30–60 in materials. Effort: 8–16 hours.

The full from-scratch receiver-tube fabrication is documented in Vol 7 § 7.4 (Work Package A). This is the same procedure used for a Vol 7 build, but Vol 6 builders sometimes choose it as a way to get a known-good receiver at minimum cost when they have the lab capability (sheet-metal forming, TIG welding) but want a Path B build for the rest of the gun. The 922(r) compliance arithmetic is unchanged — the receiver is always US-made in a Path B build, whether Option 1, 2, or 3; what changes is the cost and effort to produce it.

A Vol 6 build using Vol 7 § 7.4’s receiver fabrication is the approach a well-equipped lab builder would take for this build: the receiver is produced in a long weekend, and the rest of the build is the kit-assembly and closed-bolt FCG rework that’s distinctive to Path B.

6.5.4 Pre-1982 grandfathered open-bolt semi-auto receivers — not available for new builds

Correction, 2026-10-20. Earlier versions of this section said pre-1982 open-bolt semi-auto STEN receivers were grandfathered under a 1982 ATF rule and could be built on without the closed-bolt rework. ATF’s texts do not support that. The STEN ruling, ATF Rul. 83-5, holds the open-bolt YAC STEN MK II semi-automatic carbine to be a machine gun and contains no grandfather date (Vol 10 § 10.4).

A note on what’s not available: no ATF text was found that exempts any open-bolt semi-auto STEN, of any date, from 83-5, so there is no “grandfathered” open-bolt receiver to build on. Treat one offered for sale as a possible unregistered machine gun. For a semi-auto build, closed-bolt is the only legal posture.

6.5.5 Receiver-to-kit interface — what to verify

Whichever receiver option is chosen, verify before assembly:

Receiver-tube ID: ~35 mm. Bolt OD (~32 mm) should slide with ~1.5 mm radial clearance. Too tight binds; too loose causes timing issues.
Magazine-well slot position: ~220 mm forward of the rear end. Verify against the kit FCG housing — the housing’s magazine-well opening must align with the receiver’s magazine-well slot.
Ejection port position: opposite the magazine well at the same fore-aft location. Verify against the kit bolt — the bolt’s extractor slot must align with the ejection port at the moment of extraction.
Cocking-handle slot position: ~50 mm aft of the ejection port, running rearward 110 mm. Verify against the kit bolt’s cocking-handle pin hole — the pin must ride freely in the slot.
Trigger-group attachment holes: must align with the kit FCG housing’s attachment holes. Common interface problem — if the receiver was made for a different FCG pattern (Long Branch Mk II vs UK Mk II), the holes may not line up.
Length: ~533 mm. The kit barrel + muzzle bushing + rear cap depend on this dimension. A long receiver leaves the barrel under-protruded; a short receiver crowds the rear cap.

If any of these are off, the smart move is redrill the receiver to match the kit, not the other way around. The kit parts are 80 years old, demilled, and irreplaceable; the receiver is new and cheap.

6.6 922(r) parts-count compliance — the canonical hub treatment

This is the section that earns Vol 6 its “place in the hub where 922(r) lives” billing. Read this once and the framework applies to every imported-parts-kit build in the hub — STEN, AK-47, Sterling Mk 4 / L2A3, MP-40, FAL, etc.

6.6.1 The rule, restated

18 USC § 922(r) prohibits the assembly of a “non-importable” semi-automatic rifle or shotgun from imported parts in a configuration that would not be importable into the United States as a complete firearm. A semi-auto STEN carbine with its buttstock fitted is a rifle in the federal sense (a weapon designed to be fired from the shoulder through a rifled bore, 18 U.S.C. § 921(a)(7)), so the section reaches it on its own text; no extended interpretation is needed. Vol 10 § 10.3.3 has the reasoning.

27 CFR § 478.39 implements the rule: the assembled firearm must contain no more than 10 of the 20 listed parts in imported form. ⚠ That is the whole test: the regulation caps imported listed parts at ten. It sets no US-made quota, and the cap does not shrink when fewer than 20 of the listed parts exist on a given design. (A common restatement, that “at least 10 of the 20 must be US-made”, is only equivalent when all twenty apply.)

The 20 specified parts (the canonical 27 CFR § 478.39 list):

Table 2 — The 20 specified parts (the canonical 27 CFR § 478.39 list)

#	Part	STEN equivalent
1	Frames, receivers, receiver castings, forgings, or stampings	Receiver tube (always US-made in a Path B build — this is one US-made part)
2	Barrels	Barrel (imported in a kit build)
3	Barrel extensions	n/a for STEN (no separate barrel extension)
4	Mounting blocks (trunnions)	n/a for STEN
5	Muzzle attachments	n/a for STEN (no flash hider on Mk II; the Mk I has a flash hider — counted if building a Mk I)
6	Bolts	Bolt body (imported in a kit build)
7	Bolt carriers	n/a for STEN (the bolt is single-piece, no separate carrier)
8	Operating rods	n/a for STEN (direct blowback, no operating rod)
9	Gas pistons	n/a for STEN (no gas system)
10	Trigger housings	Trigger group housing (imported in a kit build)
11	Triggers	Trigger (imported in a kit build)
12	Hammers	Sear (the STEN’s striker — imported)
13	Sears	Sear (already counted under #12 — same part for the STEN; some compliance sheets count it once, some twice. Conservative: count once.)
14	Disconnectors	Disconnector (imported in a kit build)
15	Buttstocks	Skeleton T-frame stock or Mk V wood buttstock (imported in a kit build)
16	Pistol grips	n/a for Mk II (no pistol grip); n/a for Mk V (foregrip + buttstock, no pistol grip)
17	Forearms, handguards	Mk V foregrip (imported if Mk V); n/a for Mk II
18	Magazine bodies	Magazine body (imported: no US-made STEN magazine was found, 2026-10-18)
19	Followers	Magazine follower (imported or US, same as #18)
20	Floorplates	Magazine floorplate (imported or US, same as #18)

Of these 20 categories, 11 are applicable to a Mk II STEN build (the rest are n/a — no operating rod, no gas piston, no pistol grip, no hammer, etc.). Of the 11 applicable, the kit supplies imported versions of approximately 10: barrel, bolt, trigger housing, trigger, sear (counted once), disconnector, buttstock, magazine body, follower, floorplate. The receiver tube is always US-made (supplied by the builder) — that’s one US-made count. On the literal text (no more than 10 imported listed parts), a kit supplying ~10 imported listed parts sits at the cap, not over it. Any further imported listed part puts the build over. Builders commonly replace several imported parts with US-made ones anyway, to keep a margin against a disputed count, since whether a given STEN component counts as a listed part is not always settled. Where the margin matters, confirm the count with ATF rather than relying on a builder’s tally.

ATF’s interpretation has evolved over the decades; the conservative posture is to count every applicable category, replace enough imported parts to land at ≤10 imported, and document the count, as § 6.5.2 does for a typical Mk II kit.

6.6.2 The arithmetic — typical Mk II kit on a US receiver

A worked example: the Apex Mk II kit (with one original magazine included) built on a pre-built US-made semi-auto receiver. Out-of-box parts count:

Table 3 — A worked example: the Apex Mk II kit (with one original magazine included) built on a pre-built US-made semi-auto receiver. Out-of-box parts count

922(r) part	Source	US or imported?
Receiver tube	Pre-built US receiver	US (1)
Barrel	Apex kit	Imported (1)
Bolt	Apex kit	Imported (2)
Trigger housing	Apex kit	Imported (3)
Trigger	Apex kit	Imported (4)
Sear (hammer + sear, count once)	Apex kit	Imported (5)
Disconnector	Apex kit	Imported (6)
Buttstock (skeleton T)	Apex kit	Imported (7)
Magazine body	Apex kit	Imported (8)
Magazine follower	Apex kit	Imported (9)
Magazine floorplate	Apex kit	Imported (10)
Total applicable		11 (10 imported + 1 US)

Out-of-box, the kit-plus-receiver build has 10 imported parts and 1 US-made part — at the legal edge (≤10 imported is required; 10 is exactly the limit, and lawful). The rule requires no margin. Coming in lower is the builder’s choice, as insurance against a disputed count (§ 6.5.1) and against an imported magazine later being inserted, which adds three listed imported parts (body, follower, floorplate) to the rifle as it stands.

To build in that margin, replace two or more of the imported parts with US-made ones:

Table 4 — To build in that margin, replace two or more of the imported parts with US-made ones

Listed part to replace	Source (checked 2026-10-18)	Cost
Buttstock (skeleton T or wood)	Lab-made from steel rod or walnut (Vol 7 § 7.8 Step E7)	Material only
Trigger	Lab-made per Vol 7 § 7.5 Step B3	Material only
Disconnector	”New US made STEN disconnector”, Indianapolis Ordnance; or lab-made per Vol 7 § 7.5	$15
Sear (striker, closed-bolt build)	Lab-made per Vol 7 § 7.5, or check what the Indianapolis Ordnance SA bolt kit includes	Material only
Magazine body, follower, floorplate	No US-made STEN magazine was found. Surplus magazines are imported and add three listed parts each	—

Because no US-made magazine exists to buy, the margin has to come from the fire control and the stock. A conservative Path B build replaces buttstock + trigger + sear + disconnector = 4 imported parts replaced, leaving 6 imported parts in the rifle with a surplus magazine inserted (barrel, bolt, trigger housing, magazine body, follower, floorplate). That is comfortably below the 10-imported ceiling. Most of those four come out of the closed-bolt rework anyway (§ 6.6.2), so the margin costs little extra work.

When the closed-bolt FCG rework is also performed (§ 6.6.2), the builder fabricates or buys US-made trigger, sear/striker, and disconnector as part of that rework — those count as additional US-made replacements and drop the imported count further below the threshold. A build with the canonical 4 magazine-and-buttstock replacements + the closed-bolt FCG rework lands at 3 imported parts in an 11-applicable-part build — well under any ATF interpretive scenario.

6.6.3 What counts as “US-made”

Neither § 922(r) nor 27 CFR § 478.39 defines “US-made”. The regulation counts only imported parts from its list, so the working question for any part is whether it is an imported listed part. A part manufactured in the United States is not. The common readings are:

Whole-part manufacture in the US — unambiguous when the seller states it, as Indianapolis Ordnance does for its STEN disconnector.
Substantial-transformation rule — a foreign-origin raw material (e.g., imported steel) transformed by US labor into a finished part is US-made. A buttstock fabricated by the builder from a piece of US steel rod (or even from imported steel rod, if substantially transformed in the builder’s lab) is US-made.
Country of origin marking — for builder-fabricated parts, no country-of-origin marking is required; for purchased US-made parts, the marking on the part (or its packaging) is the documentation.
Records — retain receipts for purchased US-made parts; retain a build log for fabricated parts; retain the count documentation. ATF has historically not aggressively audited 922(r) compliance, but a builder’s own records protect against any future challenge.

A part fabricated by the builder in a US lab from raw stock is not an imported listed part: what was imported, if anything, was bar or sheet, which is not on the list. This is the posture taken for all lab-fabricated parts in the hub’s 922(r) arithmetic. “Substantial transformation” is the customs-law name for that reasoning; no ATF ruling applying it to § 478.39 was located for this volume.

6.6.4 Cross-applicable to other imported-parts-kit builds in the hub

The framework above — count the applicable parts, identify the imported subset, replace enough to land at ≤10 imported, plus whatever margin the builder wants — is identical for any imported-parts-kit build under 922(r). The Sten happens to be ~11-applicable-of-20; an AK-47 build is ~16-applicable-of-20 (more parts because of the operating rod, gas piston, bolt carrier, pistol grip, handguard, etc.); a Sterling Mk 4 / L2A3 build is ~12-applicable-of-20 (essentially the same as the Sten with minor variations in the FCG categories). The replacement-parts strategy is the same: identify the parts that are easiest to source as US-made (typically: stock, grip, handguard, magazine components, FCG components) and replace those first.

The 20 categories in 27 CFR § 478.39 are stable across the hub’s imported-parts-kit builds. Memorize the list once; apply it to each build.

6.7 Assembly procedure — from kit-and-receiver to function-tested STEN

The assembly procedure unfolds in five phases: (1) receiver-to-kit fit-up verification, (2) closed-bolt FCG rework, (3) barrel installation and headspace verification, (4) bolt and operating-mechanism installation, (5) function-test. The procedure assumes the receiver is already prepared per § 6.4 — final-machined for the magazine well, ejection port, cocking-handle slot, and trigger-group attachment holes.

6.7.1 Phase 1 — Receiver-to-kit fit-up verification (1–3 hours)

Before any rework, verify the receiver-kit interface dimensionally:

Test-fit the bolt in the receiver tube. The bolt should slide freely with ~1 mm radial clearance. Binding: dress the receiver-tube ID with a 240-grit flap wheel on a bench grinder, or replace the bolt with a slightly-smaller-OD aftermarket (rare; the WWII bolts are usually within spec).
Test-fit the FCG housing under the receiver tube. The housing should align with the receiver’s underside trigger-group attachment holes within 0.5 mm. Misalignment: redrill the receiver holes to match the housing (don’t redrill the housing — it’s the irreplaceable kit part).
Test-fit the kit barrel into the receiver tube from the muzzle end. The barrel should seat against the receiver chamber boss without forcing. Resistance: dress the receiver chamber boss with a chamfer file; the kit barrel is the reference dimension.
Test-fit the muzzle bushing. It should thread (or pin) onto the receiver tube’s muzzle end and retain the barrel.
Test-fit the rear cap. It should thread onto the receiver tube’s rear end (left-hand thread per the MoS spec; Vol 4 § 4.3.1) and retain the recoil spring.

If all five fit-ups pass, the receiver is ready for the closed-bolt FCG rework. If any fail, address the failure before proceeding — fitting issues compound downstream.

6.7.2 Phase 2 — Closed-bolt FCG rework (6–12 hours)

This is the largest single phase of the build. The kit’s FCG is configured for open-bolt operation (fixed firing pin on the bolt, sear holds the bolt rearward, trigger pull releases the bolt to fly forward and fire). For a new US semi-auto build, this must be reconfigured to closed-bolt operation (bolt rests forward with chambered cartridge, separate firing pin on the bolt, FCG striker mechanism strikes the firing pin on trigger pull).

The full closed-bolt FCG fabrication is documented in Vol 7 § 7.5 (Work Package B). For Vol 6, the procedure is the same with one simplification: the FCG housing is already supplied by the kit, so the housing itself is not fabricated — only the closed-bolt internal mechanism parts are fabricated (or purchased) and fitted to the existing kit FCG housing.

Two sub-paths:

Sub-path 1: Commercial closed-bolt FCG conversion kit.

Indianapolis Ordnance lists a “STEN MK2 SA Bolt Kit” and a “STEN MK2 SA Vertical Feed Bolt Kit”, described as semi-automatic bolt kits for the Mk 2, at $280 each (in stock, 2026-10-18). The listing does not itemise the contents, so confirm with the seller which fire-control parts are included before planning around it. An earlier draft named other kit makers and an $80–200 price; neither could be confirmed and both are removed.

Sub-path 2: Fabricate the closed-bolt FCG per Vol 7 § 7.5.

For a lab-equipped builder, fabricating the closed-bolt parts is the high-control path. Procedure per Vol 7 § 7.5 Step B3:

Modify the kit sear by grinding the bolt-engagement surface flat (the WWII sear engages the bolt’s rearward sear surface; the closed-bolt design has the sear engage a firing-pin striker, not the bolt directly).
Fabricate the firing-pin striker from 4140 round stock — small machined part that pivots in a new pocket in the kit housing and strikes the bolt’s separate firing pin from the rear. Per the Professor Parabellum Vol III drawings.
Fabricate the separate firing pin — 4140 round stock, 2.4 mm OD × 20 mm long, with a flange at the rear and a point at the front that protrudes 1.4 mm beyond the bolt face. Drill the kit bolt face to accept the firing pin (2.5 mm clearance hole through the bolt face, extending ~15 mm into the bolt body).
Fabricate the firing-pin return spring — music wire ASTM A228, ~5 mm OD coil.
Fabricate or purchase the disconnector — flat plate with pivot hole + sear-reset surface; if the build is replacing the kit disconnector for 922(r) compliance, use the fabricated US-made part here. Otherwise, the kit disconnector can be modified to interface with the new striker pivot.
Modify the kit housing by drilling a new pivot hole for the firing-pin striker (location per the Vol III drawings) and a new spring-retention feature for the striker return spring.

The kit FCG retains its WWII appearance from the outside; the internal mechanism is rebuilt for closed-bolt operation. This is the most labor-intensive part of a Path B build.

Sub-path 3: Pre-built closed-bolt FCG receiver (Option 1 of § 6.4.1 with closed-bolt features included).

If a receiver is ever bought with a new fire-control housing already integrated (none was found on sale on 2026-10-18), the kit FCG is not used in the build — it’s set aside as a parts donor (and the parts that are used from it count toward the imported parts in the 922(r) arithmetic). The closed-bolt FCG features are pre-machined; only the operating internals (trigger, sear, disconnector, striker, springs) require installation.

If Option 1 with closed-bolt features included was chosen, skip the Phase 2 fabrication and proceed directly to Phase 3.

6.7.3 Phase 3 — Barrel installation and headspace verification (2–4 hours)

Inspect the kit barrel. Bore mirror or borescope. Reject if cracks at the chamber boss or heavy pitting in the chamber.
Slip the barrel into the receiver from the muzzle end. Seat against the chamber boss.
Thread (or pin) the muzzle bushing in place. Hand-tight + light wrench.
Install the bolt (from Phase 4, but for headspace verification install it temporarily here without the recoil spring or FCG — only the bolt face against the chamber is needed).
Insert the GO gauge into the chamber. Close the bolt onto the GO gauge. The bolt must close fully without forcing — this is “headspace on GO.”
Insert the NO-GO gauge. Close the bolt. The bolt must not close fully on the NO-GO gauge — this is “headspace on NO-GO.” If the bolt closes on NO-GO, headspace is too deep — the kit barrel does not match the receiver’s chamber-boss geometry, and the build has a problem. Options: (a) reject the kit barrel and source a replacement; (b) shorten the receiver’s chamber boss to seat the barrel further forward (only if the geometry allows); (c) try a different US-made receiver. Headspace too deep is dangerous — do not proceed to firing.
Document headspace in the build log (GO closed, NO-GO did not close — record the specific gauge brand and date).

A correctly-headspaced kit barrel in a correctly-bored US receiver is the common case. Headspace failure occurs in maybe 5–10% of Path B builds, most often when the receiver and the kit were sourced from different vendors with slightly different reference dimensions.

6.7.4 Phase 4 — Bolt and operating-mechanism installation (2–4 hours)

Install the extractor in the bolt (replace it if worn; the extractor is not a listed part, so replacing it does not change the 922(r) count).
Install the cocking-handle pin and knob.
Install the recoil spring over the rear of the bolt’s spring-guide step.
Install the bolt into the receiver from the rear. With the FCG installed (next step), the bolt rides on the recoil spring and is retained by the rear cap.
Install the FCG by bolting the housing to the receiver underside. Verify the FCG bolts seat without forcing the housing-receiver interface.
Install the rear cap with light thread-locker (Loctite Blue) on the threads.
Install the stock (skeleton T-frame for Mk II; wood for Mk V). Cross-pin or bolt as appropriate.

6.7.5 Phase 5 — Function-test (1 hour)

With the build complete and before any live ammunition, perform a dry function check:

Insert an empty Sterling-pattern magazine. Magazine catch should retain it.
Cock the action by drawing the cocking handle rearward. The sear (or, in the closed-bolt design, the firing-pin striker latch) should engage and hold the action cocked.
Pull the trigger. The firing-pin striker should release and strike the firing pin (a sharp “click” is audible). The bolt does not cycle (no ammunition + no recoil = no cycle).
Manually cycle the bolt rearward. This simulates a fired round’s recoil. The disconnector should reset, the sear should re-engage, the action should be ready for the next trigger pull.
Pull the trigger again. Second strike.
Repeat 5–10 times to verify reset behavior.
Remove the empty magazine. Verify magazine release works.
Insert a snap-cap or dummy round. Verify the closed-bolt design chambers the round when the bolt is closed (the dummy round is fed from the magazine and seated against the bolt face when the cocking handle is released).

If all dry-cycle checks pass, the build is ready for the live-fire test-fire per § 6.9.

6.8 Common fitting issues and how to solve them

After hundreds of documented Path B builds in the community archive (Subguns.com, AR15.com STEN forum, Sturmgewehr.com), the recurring fitting issues fall into a small set:

FCG housing-to-receiver hole misalignment — most common. Cause: the kit FCG housing’s attachment holes were drilled for a specific factory’s WWII receiver tube (Fazakerley vs RSAF Enfield vs Long Branch had slight variation), and the US-made receiver’s holes were drilled to a different reference. Solution: redrill the receiver holes to match the housing (not the other way around).
Bolt-to-receiver fit too tight (binding) — bolt OD is at the high end of WWII tolerance, US receiver ID is at the low end. Solution: lightly polish the receiver-tube ID with a flap wheel; do not turn down the bolt OD (the bolt is the harder-to-replace part).
Bolt-to-receiver fit too loose (excessive radial clearance) — bolt OD is at the low end, US receiver ID is at the high end. Functional impact: minimal at <3 mm radial clearance; the gun functions reliably but the bolt face may not perfectly align with the chamber. Solution: accept the fit if function-testing is clean, or source a different bolt.
Barrel won’t seat against the receiver chamber boss — caused by burrs or machining swarf at the chamber boss or barrel shoulder. Solution: dress with a chamfer file, then re-test-fit.
Headspace on NO-GO closes (chamber too deep) — see § 6.6.3 Step 6. Most-common cause: mismatched kit barrel and US receiver. Solution: reject the barrel or the receiver and source a replacement; do not proceed with firing.
FCG won’t reset after a trigger pull — disconnector spring is weak (replace with a stronger music-wire spring), or the disconnector geometry is off (re-fit the disconnector against the modified sear engagement).
Closed-bolt firing-pin striker over-travels and dry-fires on the bolt face — striker travel limit is too generous; install a striker-travel-limit pin or shim per the Professor Parabellum Vol III drawings.
Magazine catch won’t retain a magazine reliably — a worn catch or a magazine with a damaged catch slot. Solution: fit a sound catch (Apex listed a surplus one at $12.95, 2026-10-18) and check it against every magazine in the working set. The catch is not a listed part.
Cocking handle binds in the receiver slot — the kit cocking handle is slightly oversized vs the US receiver’s slot. Solution: dress the cocking handle with a file, not the slot (the cocking handle is the smaller part and easier to replace).
Extractor doesn’t grip the cartridge rim — extractor spring is weak or the extractor claw is worn. Solution: replace the extractor and its spring (Indianapolis Ordnance listed both, $20 and $4, 2026-10-18). Not a listed part.

The general rule for fitting: modify the cheaper part, preserve the more-expensive or harder-to-replace part. The kit parts are 80 years old and irreplaceable; the US-made receiver and US-made replacement parts are new and modifiable.

6.9 Finishing the parts-kit build

Path B builders fall into two camps on finishing: refinish to a uniform parkerized appearance, or preserve the original WWII finish on the kit parts and finish only the US-made receiver to match.

6.9.1 Path 1 — Full refinish to uniform parkerized

Strip all metal parts to bare steel (Naval Jelly, electrolysis, or media blasting), then refinish in a single parkerizing batch per Vol 9 § 9.2. The result is a uniform-appearance build, period-correct in process if not in patina. This is the canonical Path B finish and matches the appearance of a freshly-manufactured 1944 Mk II.

Procedure per Vol 9 § 9.2:

Strip all metal parts of paint, parkerizing, oil, and surface oxide. Naval Jelly for rust removal; media blasting (aluminum oxide, 80 grit) for paint and surface treatment.
Degrease thoroughly (acetone or brake cleaner).
Submerge in manganese phosphate parkerizing solution at 95°C for 15–30 minutes until the surface is uniformly dark gray. Manganese phosphate (Du-Lite, Lauer, or homebrew) gives the dark gunmetal-gray period-correct appearance.
Rinse in hot water.
Oil with a light petroleum lubricant (Mobil 1, Outers, or any clean firearm oil).
Cure for 24 hours before assembly.

Wood parts (Mk V stock, foregrip) get boiled linseed oil per Vol 9 § 9.4.

6.9.2 Path 2 — Preserve original kit finish, match US receiver

Builders who want the kit parts to retain their WWII patina (refinishing erases 80 years of history) can leave the kit parts as-is and finish only the US-made receiver to match. This requires color-matching the parkerizing — the WWII finishes vary, so color-match the receiver to a specific kit part (typically the FCG housing) using a test strip first.

Procedure:

Test-strip parkerizing on a scrap piece of US receiver material; adjust solution temperature and dwell time to match the kit FCG housing’s color.
Strip and degrease the US-made receiver only.
Parkerize the receiver to the test-strip-confirmed color.
Light-oil the kit parts and the receiver.
Assemble.

The resulting build has a slightly variegated appearance — kit parts in their original (sometimes weathered) finish, receiver in a matching new finish. Some Path B builders prefer this “build with the past” aesthetic; some prefer the uniform fresh-1944 appearance of Path 1.

6.9.3 Cerakote alternative

Modern Cerakote (or DuraCoat) in a flat dark gray or matte black is a non-period-correct but durable alternative. Apply per the Cerakote spray-rig procedure; cure per the manufacturer’s spec. Cerakote is harder than parkerizing, more corrosion-resistant, and visually similar at conversational distance. Not period-correct; not the canonical Vol 6 finish; included here for builders who prioritize durability over historical appearance.

6.10 Test-fire, break-in, and live-fire verification

Per Vol 11’s break-in procedure (§ 11.4):

Dry function check — as in § 6.6.5 Phase 5, repeated after the gun has rested overnight.
Single-round chamber-and-fire — load one round of 9×19 NATO ball in a fresh aftermarket Sterling-pattern magazine, chamber it via the closed-bolt action (cock the gun, load magazine, ride the bolt forward). Point the muzzle in a safe direction. Fire.
Verify ejection and extraction. The case should eject cleanly through the ejection port. If the case sticks in the chamber, the extractor is weak (replace it) or the chamber is too tight (re-check with the GO gauge — should still close after firing).
5-round semi-auto — verify the closed-bolt cycle works: the bolt’s recoil from firing the first round should cycle the action, eject the spent case, feed the next round, and re-cock the closed-bolt mechanism. Pull the trigger again; second round fires.
20-round semi-auto — verify reliability across a complete magazine load.
Full magazine (32 rounds) semi-auto — verify reliability across a full magazine.
Heat-soak test — fire 3 consecutive 32-round magazines back-to-back; verify no out-of-battery firing, no double-feeds, no feed-lip pathology. If any of these occur with a fresh aftermarket magazine, the issue is in the FCG or the receiver geometry, not the magazine.
Clean and inspect — disassemble after the heat-soak test, clean per Vol 11 § 11.7, inspect for any visible wear, fit, or damage. Pay attention to the closed-bolt firing-pin and striker — early-build fitting can show as accelerated wear here.

A Path B build that passes the heat-soak test is reliable for ordinary range use. The WWII surplus Sten magazines (if any are included in the kit) are not for range use — keep them as collector items and shoot with modern aftermarket only.

6.11 Cost and time-on-task summary

Table 5 — 6.10 Cost and time-on-task summary

Line item	Low	High	Notes
Parts kit (Apex / Sarco Mk II)	$200	$500	Complete kit; partial kits cheaper but missing parts will need to be sourced separately
US-made semi-auto receiver	$30	$400	$30–60 scratch (Vol 7 § 7.4); $150 80%; $250–400 pre-built
Closed-bolt FCG kit OR fabricated parts	$50	$200	$50 fabricated from raw stock; $80–200 commercial conversion kit
922(r) US replacement parts (stock, trigger, sear, disconnector)	$15	$100	Lab-made, or the $15 US-made disconnector (Indianapolis Ordnance)
2 modern Sterling-pattern magazines	$50	$80	$25–35 each; buy two minimum
Headspace gauges (GO + NO-GO, 9×19)	$40	$60	One-time tooling investment
Parkerizing supplies (if refinishing)	$30	$80	Du-Lite or Lauer manganese phosphate + 5-liter tank
Form 1 SBR tax stamp (if barrel <16″)	$200	$200	One-time; 6–10 month wait
Miscellaneous (cleaning, oil, springs, pins)	$20	$50
Total	$670	$1,670	Without SBR: $470–1,470
Time (first build)	15 hr	30 hr	Subsequent builds: 8–15 hr

The cost spread is wide because of three big variables: parts-kit price (depends on market conditions and condition grading), receiver choice (3× spread), and whether the build is configured as an SBR. The time spread is driven by experience level and by whether the builder fabricates or purchases the closed-bolt FCG.

For a typical lab-builder Path B build with a mid-range Apex kit, a Vol 7 § 7.4-fabricated receiver, fabricated closed-bolt FCG, lab-made replacement parts, and Form 1 SBR posture: ~$650 + ~20 hours.

6.12 Stretch builds — Mk V parts kit and NFA Class III option

6.12.1 Mk V parts-kit build (wood furniture + No-4 sights)

A Mk V kit (Sarco and Apex carry these intermittently) has the same parts-tree as a Mk II but adds:

Mk V wooden buttstock (replaces the skeleton T-frame) — counted as a 922(r) buttstock.
Mk V wooden foregrip — counted as a 922(r) forearm/handguard.
Lee-Enfield No-4 rear sight + adjustable front blade — not 922(r) counted.
Bayonet lug on the muzzle bushing — not 922(r) counted (some 922(r) sheets list “bayonet lug” but it’s not in the 27 CFR § 478.39 list of 20 parts).

The Mk V build is slightly more constraining on 922(r) compliance because the wood furniture adds a second buttstock-or-handguard count category that’s typically imported in the kit (~12 applicable parts, ~11 imported out-of-box). Replacement strategy: US-made buttstock + US-made foregrip + the standard magazine package = 6 replacements, leaving ~5 imported. Sources: the lab can make the wood (Mk V profiles from the original drawings); no maker of US-made Mk V furniture was confirmed for this volume.

Cost premium vs Mk II: $50–150 (kit) + $100–200 (US wood furniture) = $150–350 over a Mk II build.

6.12.2 NFA Class III pre-86 registered transferable receiver (Path A + Path B kit parts)

If the builder has access to a pre-86 registered transferable Sten receiver (via a Class III SOT or by direct purchase from a Class III dealer — Vol 5 has the procedure), the Path A registered receiver can be combined with a Path B kit. The result is a select-fire Sten built on a registered NFA receiver, using kit parts for the bolt, FCG (in the original WWII open-bolt configuration — no closed-bolt rework required, because the registered receiver is a machine gun and already registered as one, so ATF Rul. 83-5’s open-bolt classification is no obstacle), barrel, stock, and magazines.

Legal posture: the registered receiver is the gun for NFA purposes; the kit parts are service parts. ATF will recognize a registered Sten with kit-sourced bolt, FCG, and barrel as long as the registration is properly recorded and the receiver remains in its registered configuration. The kit’s open-bolt FCG and the kit’s bolt with fixed firing pin are appropriate for the registered receiver — this is the only Path B variant where the WWII parts are used as-built.

Cost: $10–25K for the registered receiver (per Vol 5), plus $200–500 for the kit, plus minimal additional cost (no closed-bolt rework needed; minor receiver-kit fitting only). This is the cheapest path to a select-fire transferable Sten if a registered receiver can be sourced — the kit drops in, the Vol 6 fit-up and finish steps apply, and Path C’s full-from-scratch labor is avoided.

922(r) does not apply to NFA-registered machine guns (which are not “non-importable” semi-automatic rifles in the relevant statutory sense). The Vol 6.5 parts-count arithmetic is unnecessary for this build path.

Cross-references: Vol 5 (NFA acquisition procedure), Vol 10 § 10.2 (NFA posture), Vol 11 § 11.6 (select-fire range procedure).

6.13 Failure modes and Vol 11 cross-references

Each Path B failure mode has a Vol 11 cross-reference for diagnosis + remedy:

Out-of-battery fire — closed-bolt firing-pin striker is over-traveling; reduce striker travel limit. Vol 11 § 11.5.
Light primer strikes — firing-pin spring weak or firing-pin tip short; replace spring or fit longer pin. Vol 11 § 11.5.
Stovepiped cases — extractor weak or wrong angle; replace extractor. Vol 11 § 11.5.
Failure to feed — magazine feed-lip pathology (use fresh aftermarket Sterling-pattern); or receiver-to-barrel ramp geometry. Vol 11 § 11.3.
Failure to extract — chamber too tight, extractor weak, or chamber-to-receiver geometry off. Vol 11 § 11.5.
FCG fails to reset — disconnector spring weak or disconnector binding. § 6.7 above + Vol 11 § 11.5.
Closed-bolt slam-fire (double-tap on single trigger pull) — firing-pin striker latch failed to re-engage after first shot; rework the striker pivot or replace the latch. Vol 11 § 11.5.
Headspace failure (NO-GO closes after some firing) — chamber wear or bolt-face wear; re-verify with gauges; if confirmed, the gun needs receiver service or replacement. Stop firing. Vol 11 § 11.5.
Magazine-catch failure — worn catch or damaged magazine slot; fit a sound catch and check every magazine. § 6.7 above.

6.14 References (Vol 6)

Vol 4 of this series — Engineering & Reference Data; the dimensional source for receiver-to-kit fit-up and the closed-bolt FCG rework reference.
Vol 5 of this series — Build Path A (NFA Class III). The cross-reference for the § 6.11.2 NFA stretch build.
Vol 7 of this series — Build Path C (from-scratch). The cross-reference for the closed-bolt FCG fabrication (§ 6.6.2 sub-path 2) and the from-scratch receiver option (§ 6.4.3).
Vol 9 of this series — Materials & Finishing; the surface-treatment authority for § 6.8.
Vol 10 of this series — Legal & Regulatory Posture; the legal authority for 922(r), ATF Rul. 83-5 (open-bolt classification), SBR posture, and the per-state map.
Vol 11 of this series — Live-Fire Operation & Use; the test-fire procedure for § 6.9 and the failure-mode diagnosis for § 6.12.
18 USC § 922(r) — the statute prohibiting assembly of non-importable firearms from imported parts.
27 CFR § 478.39 — the regulation implementing § 922(r); specifies the 20-parts list.
Apex Gun Parts. STEN category and search listings, https://www.apexgunparts.com/machine-guns/sub-machine-guns/sten.html, consulted 2026-10-18.
Indianapolis Ordnance. STEN Mk 2 parts and Sterling Mk 4 pages, https://indianapolisordnance.com/parts/STEN-MK2-Parts, consulted 2026-10-18.
Professor Parabellum. Practical Scrap Metal Small Arms, Volume III: The DIY STEN Gun. (Wikimedia Commons, CC BY 1.0 PDF.) The dimensional source for the closed-bolt FCG layout (§ 6.6.2 sub-path 2).
Wise Lite Arms. wiselitearms.com redirects to wlarms.com, which did not resolve on 2026-10-18.
Sarco Inc., Numrich Gun Parts. Alternate parts-kit and individual-parts vendors.
ATF Rul. 83-5 (1983) — the YAC STEN MK II open-bolt semi-auto carbine is a machine gun; with 82-2 and 82-8 (1982), the open-bolt classifications. The legal foundation for the closed-bolt rework requirement (Vol 10 § 10.4).
Subguns.com, AR15.com STEN forum, Sturmgewehr.com STEN board. Community-archive build threads; the experiential reference for common fitting issues (§ 6.7).