Sub-Assemblies & Components

Per-part mechanics one layer below Vol 4 — receiver tube, bolt, FCG, magazine, stock

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

This volume goes one layer deeper than Vol 4 (Engineering & Reference Data). Where Vol 4 catalogs the dimensional reference for each part, Vol 8 explains how each part works — the function of the geometry, the engineering rationale tied to the 1941 design brief, the per-variant variation, and the common-failure-mode awareness for a builder who wants to understand why each part is the way it is.

The volume is organized by sub-assembly, in roughly fore-to-aft order: receiver tube (§ 8.2), bolt + firing pin (§ 8.3), trigger group housing + FCG (§ 8.4), barrel + chamber (§ 8.5), magazine (§ 8.6), stock variants (§ 8.7), and miscellaneous small parts (§ 8.8). Each section ends with a brief “common amateur fabrication mistakes” callout for builders who are about to make that part.

This volume is useful in two contexts:

Before fabricating a part (Vol 7 builders) — read the relevant § to understand why the part is the way it is before cutting metal.
When diagnosing a malfunction (Vol 11 § 11.5 cross-references) — read the relevant § to understand the mechanism behind a failure mode.

8.1 The Sten as a system — how the parts work together

Before drilling into per-part detail, the overall system is worth restating. The Sten is a blowback-operated, open-bolt, fixed-firing-pin submachine gun firing 9×19 from a 32-round side-feed magazine. The chain of events from cocked-and-ready to ejection:

Cocked state: bolt is held to the rear by the sear. Recoil spring is compressed behind the bolt. Magazine is inserted, top round positioned at the feed lips. Trigger has not been pulled.
Trigger pull: trigger pivots, depresses the sear, sear releases the bolt.
Bolt flight forward: recoil spring drives bolt forward. As it moves, the bolt face encounters the top round in the magazine, picks it up, and chambers it. The bolt’s fixed firing pin strikes the cartridge primer at the moment of full chambering.
Cartridge fires: ~36,500 psi peak chamber pressure drives the bullet out the barrel and (simultaneously) drives the case rearward against the bolt face. The bolt has high inertia (~590 g) — its rearward acceleration is slow relative to the bullet’s forward acceleration, which gives the bullet time to clear the muzzle before the case starts to extract.
Bolt rearward stroke: bolt moves rearward, compressing the recoil spring. The extractor on the bolt face holds the case rim; the case is dragged out of the chamber as the bolt moves.
Ejection: as the bolt reaches the rear of the receiver, the spent case clears the ejection port and is thrown clear. The bolt is now at the rear of its travel.
Sear engagement (in semi-auto): the sear catches the bolt’s sear surface; the bolt is held to the rear. In full-auto, the sear is held depressed by the trigger; the bolt is allowed to fly forward again immediately, repeating the cycle.
Next round picks up: as the bolt flies forward (auto-cycle), the next round is presented at the feed lips and picked up by the bolt face.

The cycle continues at the cyclic rate (~550 rpm full-auto) until the trigger is released (semi-auto) or the magazine is empty (no round to chamber → bolt stops with nothing to fire).

This simple blowback approach was the 1941 design choice for production economy. There are no gas-piston systems, no locked breech, no operating rods, no bolt carriers. The cartridge’s chamber pressure pushes the bullet forward and the bolt rearward simultaneously, and the bolt’s mass alone (against the recoil spring) is the only force resisting the rearward push. This is why bolt mass matters — at ~590 g, the Sten’s bolt is the heaviest of the WWII subguns in 9×19, and the heavy bolt is the gun’s safety against pre-chamber pressure release.

8.2 The receiver tube — the magic and the failure mode

8.2.1 What it does

The receiver tube is the structural backbone of the gun:

Houses the bolt on a sliding fit (32 mm bolt OD in 35 mm ID, ~1.5 mm radial clearance).
Aligns the chamber at the muzzle end via the chamber boss.
Retains the magazine via the magazine well.
Guides the cocking handle in its slot.
Provides the structural axis along which all other parts are aligned.

It does not contain the chamber pressure — the barrel does that. The receiver tube experiences only the bolt’s recoil thrust transferred via the recoil spring + rear cap, which is a small fraction of the chamber pressure.

8.2.2 The wartime engineering choice

The receiver tube is rolled and welded mild-steel sheet, approximately 38 mm OD × 35 mm ID × 1.5 mm wall × 533 mm length. This geometry was chosen for production economy:

Sheet stock can be sourced anywhere — UK sheet-metal shops in 1941 had no shortage of 1.5 mm cold-rolled mild steel.
Roll-and-weld is a single-operation process — a sheet metal worker with a roller and a welding station can produce 50–100 receiver tubes per shift.
Longitudinal seam allows the tube to be unwrapped flat for shipping if needed (it wasn’t; but the geometry permits it).
No machining of the tube interior — the rolled-on-mandrel forming establishes the ID directly, without requiring lathe boring.

The result is a receiver tube that costs essentially the same as the sheet stock plus a few minutes of labor. Compare this to the Thompson submachine gun’s receiver, which was machined from a 4-pound steel forging at 60+ minutes of machinist time per unit. The Sten’s cost advantage was structural to its 1941 design brief.

8.2.3 Per-variant variation

Mk I and Mk II (UK): standard wrapped-and-welded tube as described.
Mk III (Lines Brothers Tunbridge): one-piece stamped receiver — the receiver is not a separate tube but the upper half of a stamped sheet-metal shell that includes the bolt-guide track, the magazine well, and (forward) the chamber-boss area. This is the simplest variant in the Sten family; it eliminates the weld seam entirely.
Mk V (RSAF Enfield): standard wrapped-and-welded tube with refinements — slightly thicker wall (1.6–1.8 mm), better weld seam preparation, machined exterior for bayonet-lug and sight-base attachment.

8.2.4 Common amateur fabrication mistakes

Insufficient stress-relief after welding — the as-welded receiver tube has residual welding stress that, under heat-cycling, can propagate cracks at the seam. Sub-critical anneal (Vol 9 § 9.5.5) is recommended.
Off-spec wall thickness — substituting 1.2 mm sheet (commonly available but thinner than spec) under-builds the receiver and reduces fatigue life. Stick with 1.5 mm spec.
Asymmetric weld bead — uneven bead deposition produces an out-of-round tube; verify circularity post-weld at multiple stations.
Seam orientation — the seam should be at the top-left per the MoS spec (Vol 4 § 4.3.1) so it doesn’t fall on a high-stress surface or interfere with cutout machining. Random orientation works mechanically but visually mismatches a period-correct build.
Burrs on the magazine-well cutout — the magazine-well opening must be smooth-edged for reliable magazine insertion; burrs hold up the magazine and cause feed issues.

8.3 The bolt and the fixed firing pin

8.3.1 What it does

The bolt is the cartridge-driving and case-extracting element:

Bolt face presents the cartridge primer to the fixed firing pin at chambering — this is the unique Sten characteristic.
Bolt OD rides in the receiver tube on a sliding fit.
Bolt mass (~590 g) provides the inertia that delays rearward stroke long enough for chamber pressure to drop.
Extractor on the bolt face grips the case rim during the rearward stroke.
Cocking-handle pin transmits the cocking force from the cocking handle to the bolt.
Sear engagement surface on the bolt’s underside catches the sear when the action is cocked.

8.3.2 The fixed-firing-pin pattern

The Sten’s signature mechanical feature: the firing pin is integral to the bolt face, machined as a small projection at the center of the bolt face, with no spring, no striker, no separate firing-pin mechanism.

Consequences:

Open-bolt operation is mandatory — the bolt must be held rearward and slammed forward to chamber and fire in one event. There is no way to “cock the gun and wait” with a fixed firing pin.
The bolt’s forward inertia is the firing impulse — the bolt mass times its forward velocity at chambering determines the primer-strike energy. This works because the bolt is heavy and the recoil spring is moderately stiff; the result is reliable primer ignition with no separate firing-pin spring needed.
The fixed firing pin cannot be replaced separately — if it wears or chips, the entire bolt is the replaceable unit.
Closed-bolt operation requires redesign — Vol 6 § 6.6.2 and Vol 7 § 7.5 cover the closed-bolt FCG rework, which adds a separate firing pin to the bolt face and a striker mechanism in the FCG. The fixed firing pin is removed (or, in a new-build bolt, never machined).

8.3.3 Bolt mass and cyclic rate

The bolt mass determines the gun’s cyclic rate in full-auto. The Sten’s 590 g bolt produces a cyclic rate of approximately 550 rpm at standard 9×19 NATO ball pressure. Variants:

Lighter bolt → higher cyclic rate, harder primer strike, more sensitivity to ammunition pressure variation.
Heavier bolt → lower cyclic rate, gentler primer strike, more reliable at low-pressure ammunition.

The wartime production was deliberately conservative with bolt mass — 590 g is on the heavy side of what a 9×19 SMG bolt needs to be. The Sten cycles reliably at sub-pressure ammunition because the heavy bolt has enough inertia to ride out variations in chamber pressure.

8.3.4 Bolt-face case-hardening

The bolt face contacts the cartridge primer at firing-pressure impacts (each shot). Without case-hardening, the bolt face gradually erodes and the firing-pin tip rounds over → light primer strikes (Vol 11 § 11.5.2). The wartime spec called for case-hardened bolt faces; modern equivalents are documented in Vol 9 § 9.5.1.

8.3.5 Per-variant variation

Mk II UK (RSAF Enfield, BSA, Fazakerley): standard 590 g bolt, machined from plain carbon steel, case-hardened bolt face.
Long Branch Mk II (Canada): standard 590 g bolt; some sub-batches have slightly different case-hardening procedure.
Mk V (RSAF Enfield): refined bolt with smoother surfaces and tighter tolerances; functionally identical to Mk II.

8.3.6 Common amateur fabrication mistakes

Undersized OD (>0.2 mm under spec) — bolt is loose in the receiver; can cause out-of-battery fire from off-axis chambering.
Oversized OD (>0.1 mm over spec) — bolt binds in the receiver; reliability suffers as fouling builds up.
Insufficient bolt-face case-hardening — bolt face erodes after 500–1000 rounds; light primer strikes develop.
Wrong firing-pin protrusion — too short → light primer strikes; too long → out-of-battery fire risk. Spec is ~1.4 mm protrusion beyond bolt face.
Missing extractor slot — bolt won’t extract cases; chamber becomes a case-stuck-in-chamber failure mode.
Off-center firing pin — primer strike is off-center, leading to reliable misfires. Center the firing pin within ±0.2 mm of the bolt face center.

8.4 The trigger group housing and FCG

8.4.1 What it does

The FCG (Fire Control Group) housing is the structural shell that contains the trigger, sear, disconnector, magazine catch, and (for select-fire variants) the fire-mode selector. It bolts to the receiver tube’s underside and presents:

The trigger (operated by the shooter).
The magazine well opening (aligned with the receiver-tube cutout for magazine insertion).
The sear (engages the bolt to hold it cocked).
The disconnector (in semi-auto: resets between shots; in full-auto: held in non-disengagement position by the selector).
The fire-mode selector (Mk II UK: selects semi-auto or full-auto; absent on semi-auto-only variants).

8.4.2 The mechanical action

In open-bolt full-auto (Path A, Vol 5):

Trigger pull depresses the sear via the trigger-sear linkage.
Sear releases the bolt; bolt flies forward, chambers, fires, recoils, returns.
As long as the trigger is held, the sear stays depressed → bolt cycles repeatedly at the cyclic rate.
Trigger release: sear pivots up; next bolt-rearward-cycle catches the sear; bolt is held cocked.

In open-bolt semi-auto (selector position; Path A select-fire):

Trigger pull depresses the sear; bolt cycles once.
Disconnector intercepts: as the bolt cycles rearward, the disconnector engages and forces the trigger-sear linkage to disengage even if the trigger is held.
To fire the next shot, the trigger must be released and re-pulled. The disconnector then resets.

In closed-bolt semi-auto (Path B/C, post-1982):

Bolt is forward at rest. Separate firing pin in the bolt; firing-pin striker mounted in the FCG.
Trigger pull releases the firing-pin striker (not the bolt) via a sear-striker engagement.
Firing-pin striker pivots forward and strikes the separate firing pin in the bolt → primer ignition.
Cartridge fires; chamber pressure drives the bolt rearward.
Bolt-rearward motion cocks the firing-pin striker (via a cam ramp on the bolt’s underside).
As the bolt returns forward (after recoil-spring return), it chambers the next round.
Bolt is now forward, ready for next trigger pull. Disconnector ensures the trigger must be released before the next pull.

The closed-bolt semi-auto design is mechanically different from the open-bolt original (different firing-pin mechanism, different sear engagement, no full-auto capability by design) but occupies the same FCG-housing volume. Vol 6 § 6.6.2 documents how the kit FCG housing is reworked to accept the closed-bolt mechanism.

8.4.3 Per-variant variation

Mk II UK (RSAF Enfield, BSA, Fazakerley): stamped sheet steel FCG housing, ~16-gauge. Includes the iconic angled fire-mode-selector slot.
Long Branch Mk II (Canada): cast aluminum FCG housing. Visually identical at conversational distance to the stamped UK version; mechanically equivalent. Some sub-batches have hairline cracks at the magazine-catch pivot — inspect with magnifier.
Mk III (Lines Brothers): similar stamped sheet FCG; tube and housing are welded as a one-piece receiver.
Mk V (RSAF Enfield): refined FCG with improved sear-block safety (vs Mk II’s cocking-handle-slot safety). Slightly larger trigger guard for gloved use.
Modern Vol 7 from-scratch: typically CNC-machined 6061-T6 aluminum for cleaner appearance and faster fabrication.

8.4.4 The Mk II UK fire-mode selector

The Mk II’s fire-mode selector is a horizontal slot in the FCG housing, with a pin/lever that the shooter pushes forward (full-auto) or rearward (semi-auto). The lever mechanically engages or disengages the disconnector spring.

Quirks:

Selector position-by-feel is unreliable in low-light conditions. The Mk V improves on this with a more positive detent.
The selector can be pinned/welded to semi-auto-only on a registered transferable (Path A) for builders who want a registered SMG that fires only semi-auto. This is uncommon but possible; the gun remains an NFA machine gun (the registered status is unchanged by mechanical disablement of full-auto).

8.4.5 Common amateur fabrication mistakes

Misaligned trigger-sear linkage — trigger pull doesn’t fully depress the sear; gun fails to fire.
Over-engaged sear — sear engages the bolt too deeply; trigger pull requires excessive force, or the sear may not release reliably under cyclic-fire conditions.
Weak disconnector spring — disconnector fails to reset between shots; gun double-feeds or doubles.
Stamped-sheet housing too thin — replicating the WWII gauge from modern thin sheet metal produces a flimsy housing; stay at 16-gauge minimum.
Closed-bolt striker over-travel — striker has too much travel; can strike the firing pin before the bolt is fully forward → out-of-battery fire. Install a striker-travel limit.

8.5 Barrel, chamber, and lockup geometry

8.5.1 What it does

The barrel is the bullet-accelerating element. The chamber is the cartridge-holding pocket. Together they:

Accept the cartridge at the chamber mouth (where the bolt face delivers it).
Constrain the cartridge during firing (the chamber’s circumferential walls).
Accelerate the bullet down the bore length under chamber pressure.
Direct the bullet out the muzzle with spin imparted by the rifling.

The Sten’s barrel is simple in design — straight cylindrical bore, simple chamber, conventional rifling, no flutes or other complexity. This is part of the 1941 design economy.

8.5.2 Barrel-to-receiver lockup

There is no rotating-bolt lockup on a Sten — the barrel is fixed to the receiver tube via a threaded or pinned chamber-boss arrangement. The bolt simply rides forward against the cartridge in the chamber; chamber pressure pushes the cartridge against the chamber walls (which contain it) and against the bolt face (which is held forward by the recoil spring and bolt mass). The bolt does not lock to the barrel — it’s held by mass and spring alone (simple blowback).

This is why bolt mass matters (§ 8.3.3) and why the 9×19 cartridge is at the practical upper edge of straight-blowback operation. Heavier cartridges (.45 ACP, 7.62×25 Tokarev, 9×23 Largo) require more bolt mass or some form of locked-breech operation; the Sten’s design was carefully tuned for 9×19 at British / NATO pressure.

8.5.3 Chamber geometry

The 9×19 chamber follows the SAAMI 9×19 Parabellum spec:

Chamber length: 19.15 mm (case length).
Chamber diameter at the case mouth: ~9.65 mm.
Chamber diameter at the case head: ~9.93 mm (slight taper for case extraction).
Throat diameter (at bullet seating): ~9.65 mm.
Headspace: cartridge case mouth seats on the chamber shoulder; bolt face stops the case head at ~19.20–19.40 mm depth.

The chamber-mouth chamfer feeds the cartridge into the chamber during chambering. A sharp chamber mouth causes failure-to-feed (Vol 11 § 11.5.4); a properly-chamfered mouth feeds reliably.

8.5.4 Rifling

The Sten’s barrel rifling is conventional:

4-groove right-hand twist, 1:10 inch (1 turn per 10 inches of bore length).
Bore diameter: 0.355″ (9.0 mm) groove-to-groove.
Land diameter: 0.347″ (8.81 mm).
Rifling depth: ~0.004″ (0.10 mm).

These dimensions match standard 9×19 sporting and military rifling. A modern pre-rifled barrel blank from any reputable US vendor (E.R. Shaw, Green Mountain, McGowen) is dimensionally compatible.

8.5.5 Per-variant variation

Mk II, Mk III, Mk V: standard 197 mm (7.75″) barrel for Mk II/III; ~196 mm for Mk V. All chambered for 9×19; all use the same rifling spec.
Mk I: slightly longer barrel (~205 mm) with a finned cooling jacket and flash hider. Period-correct for the early machined Mk I; uncommon in modern builds.
Mk VI (suppressed): integral suppressor with a longer overall length but the same effective barrel length to the suppressor entry. The suppressor itself is NFA-registered separately from the gun.

8.5.6 Common amateur fabrication mistakes

Sharp chamber mouth (no chamfer) — failure-to-feed at every shot.
Off-center chamber — bullet hits the chamber wall on chambering; reliability fails immediately. Use a floating reamer holder (Vol 7 § 7.7 Step D3).
Wrong headspace — see § 8.5.3 chamber depth; verify with GO + NO-GO gauges.
Improperly crowned muzzle — gas escape is uneven; accuracy degrades. Use an 11° crowning cutter.
Off-axis barrel-to-receiver fit — barrel doesn’t align with the bolt; chambering fails. Verify with a barrel-axis-to-bolt-axis test (bore-sight tool or bolt-running-in-receiver alignment check).

8.6 The magazine — Lanchester origin and the feed-lip problem

Vol 11 § 11.3 owns the range-day view of the magazine; Vol 8 § 8.6 owns the engineering view.

8.6.1 What it does

The 32-round magazine holds the cartridges and presents them to the bolt face one at a time as the bolt cycles. The magazine:

Holds the cartridges in a single-feed column (single-stack from inside-out, with both rims aligned to the same feed-lip presentation).
Spring-loaded follower pushes the cartridge stack upward against the feed lips.
Feed lips constrain the top cartridge until the bolt face engages and pushes it forward into the chamber.
Catch geometry at the bottom retains the magazine in the well.

8.6.2 The Lanchester origin

The Sten magazine is directly derived from the Lanchester carbine magazine, which is itself derived from the German MP 28 (Hugo Schmeisser’s 1928 commercial subgun). The lineage:

1918 MP 18 (Schmeisser, end of WWI): drum magazine derived from the Pistole 08 “snail” magazine. 32-round capacity.
1928 MP 28 (Schmeisser, commercial successor to the MP 18): straight box magazine with single-feed; 32-round capacity. The first practical SMG straight magazine.
1941 Lanchester carbine (Sterling Engineering, UK): the Lanchester was the British Navy’s emergency SMG procurement before the Sten; explicitly copied the MP 28 including the magazine design. Compatible with the German MP 28’s magazines.
1941 Sten: the Sten magazine is a slightly-cheaper-to-manufacture Lanchester magazine — same single-feed geometry, same 32-round capacity, but stamped sheet steel instead of the Lanchester’s machined construction.
Post-war Sterling Mk 4 / L2A3 (Sterling Engineering): the Sterling’s magazine is the mature evolution of the same Lanchester-derived design, with refined feed-lip geometry, curved 34-round body for better feed reliability, and tighter manufacturing tolerances.

This Sten/Lanchester/Sterling lineage means modern Sterling-pattern magazines (Bear Arms, ASP, military surplus Sterling Mk 4) work in a Sten with minor (or no) magazine-catch adjustment. This is the canonical Vol 6 / Vol 11 magazine recommendation.

8.6.3 The feed-lip problem

Single-feed magazines depend on tight feed-lip geometry tolerance for reliability. The wartime production used stamped sheet-steel feed lips welded into the magazine body, with wide per-magazine variation in lip parallelism, lip angle, and lip thickness. Vol 11 § 11.3.1 has the range-day consequence: a box of 100 WWII Sten magazines has approximately 20–30% reliable, 30–40% marginal, 30–50% unreliable.

The feed-lip pathology is the gun’s worst feature and has been documented since 1941 — UK soldiers in WWII routinely cited magazine unreliability as the Sten’s primary failing. The Sterling Mk 4’s redesign (curved magazine, refined lips) was specifically driven by this feedback.

For a modern build, use Sterling-pattern aftermarket magazines (Bear Arms, ASP, post-war Sterling military surplus) and treat the WWII original magazines as collector items.

8.6.4 Magazine spring and follower

Follower: stamped sheet steel cap that rides up the cartridge stack as rounds are stripped from the top. WWII followers are sometimes worn at the cartridge-contact surface; aftermarket replacements are dimensionally identical.
Spring: helical compression spring, ~10–12 mm OD, ~80 mm free length, music wire ASTM A228 in modern production. The wartime springs sometimes fatigue after 80+ years; replace with modern equivalents (Bear Arms or similar).
Floorplate: stamped sheet steel; retains the spring and follower; opens (typically by removing a small pin) for cleaning.

8.6.5 Common amateur fabrication mistakes

For Vol 7 builders fabricating their own magazines (uncommon — the canonical recommendation is to buy Sterling-pattern aftermarket):

Feed-lip angle wrong — round presents at wrong angle; failure-to-feed.
Feed-lip parallelism off — one side higher than the other; rounds skew sideways on presentation.
Insufficient spring tension — rounds don’t lift to feed height fast enough; bolt arrives before the round is positioned.
Magazine catch geometry wrong for the receiver well — magazine doesn’t lock or falls out under firing.

The strong recommendation for Vol 7 is to use commercial Sterling-pattern magazines, not fabricate from scratch. Magazine fabrication is more difficult than receiver fabrication.

8.7 Stock variants

8.7.1 What it does

The stock provides the shooter’s contact point for shouldering and aiming. Sten stocks are intentionally simple — the gun is a submachine carbine, not a precision rifle, and the stock geometry doesn’t materially affect accuracy beyond providing a stable shoulder contact.

8.7.2 Per-variant variation

Mk II skeleton T-frame: 9.5 mm steel rod, bent into an inverted T and welded at the joint. ~210 mm total length; attaches to the receiver-tube rear lug via a cross-pin. The most-iconic Sten stock — minimal, cheap, period-correct.
Mk II wire-loop: alternative Mk II stock — a single loop of 9.5 mm rod, attached at one end. Slightly more compact than the skeleton T; less rigid.
Mk II solid bar stock (some sub-batches): solid steel bar in a T or rectangular profile. Heavier; rare.
Mk III “stamped” stock: integral with the Mk III stamped receiver; not a separate stock.
Mk IV paratrooper folding stock: prototype design with a side-folding stock for paratrooper use. Never volume-produced.
Mk V wooden buttstock: walnut, finished with boiled linseed oil. The “respectable Sten” stock that gives Mk V its distinctive appearance.
Mk V wooden foregrip: walnut foregrip clamped under the barrel, ahead of the magazine well. Pairs with the wooden buttstock.

8.7.3 Modern Vol 7 fabrication

Vol 7 § 7.8 Step E7 covers the Mk II skeleton T-frame stock fabrication: bend 9.5 mm 1018 round bar to the T-shape, TIG-weld the joint, and attach via the receiver-tube rear lug.

For Mk V wood furniture (Vol 7 § 7.12.1 stretch goal): CNC-router walnut blanks from hardwood stock per the Vol 9 § 9.7 wood-finishing procedure.

8.7.4 Common amateur fabrication mistakes

Weak weld at the T-joint — the Mk II skeleton T-stock takes shoulder force during firing; a weak weld can fail. Verify with a destructive test on a scrap stock first.
Wrong rod diameter — 9.5 mm is the spec; thinner rod feels flimsy; thicker rod is hard to bend.
Mk V wood splitting at the wrist — bad grain orientation in the blank; verify grain runs along the stock length.

8.8 Miscellaneous small parts

The Sten has a number of small parts that don’t fit into the larger sub-assemblies but warrant brief treatment:

8.8.1 Muzzle bushing

Small lathe-turned part that threads (or pins) into the front of the receiver tube and retains the barrel. Cut from 1045 or 4140 bar at ~38 mm OD × 15 mm thick; bored for the barrel fit; threaded for the receiver-tube interior thread. Vol 7 § 7.8 Step E1 has the fabrication procedure.

8.8.2 Rear cap

Threaded plug at the rear of the receiver tube that retains the recoil spring. Left-hand thread per Vol 4 § 4.3.1 — this prevents the cap from loosening under the spring’s torque-as-it-compresses. Vol 7 § 7.8 Step E3.

8.8.3 Cocking handle

Small steel knob threaded or pinned onto a cocking-handle pin that protrudes from the bolt through the receiver-tube slot. Allows the shooter to manually cycle the bolt. Mk II has a knob-and-pin design; later variants have a slightly different shape.

8.8.4 Extractor

A small spring-steel claw, riveted or pinned into the bolt face’s extractor slot. Grips the case rim during the rearward stroke. Wear-prone part; replace if the gun develops stovepipes (Vol 11 § 11.5.3).

8.8.5 Sights

Mk II: fixed front blade (welded to the receiver-tube top) + flip-aperture rear sight (~100 yd setting) on the receiver-tube top.
Mk V: Lee-Enfield No 4 rear sight (adjustable aperture, 100–1000 yd) + adjustable front blade. The rear sight is a sourced-from-No-4-rifle part, not fabricated from scratch.

8.8.6 Bayonet lug (Mk V)

Welded to the muzzle bushing; accepts a No 4 bayonet or a custom Sten-style bayonet. Period-correct for Mk V; absent from Mk II and earlier.

8.8.7 Magazine catch

Spring-loaded lever in the FCG housing that retains the magazine. Mk II catch geometry differs slightly from Sterling-pattern aftermarket magazine geometry — if using Sterling magazines, install a Bear Arms-pattern catch.

8.9 Cross-reference summary

For builders looking at a specific part:

Table 1 — For builders looking at a specific part

For this component…	Read these sections
Receiver tube	§ 8.2 + Vol 4 § 4.3 + Vol 7 § 7.4
Bolt	§ 8.3 + Vol 4 § 4.4 + Vol 7 § 7.6 + Vol 9 § 9.5.1
FCG housing (Mk II UK stamped)	§ 8.4 + Vol 4 § 4.5 + Vol 9 § 9.1
FCG housing (CNC-machined aluminum)	§ 8.4 + Vol 4 § 4.5 + Vol 7 § 7.5
Closed-bolt FCG (Path B/C semi-auto)	§ 8.4.2 + Vol 7 § 7.5 + Vol 6 § 6.6.2 + Vol 10 § 10.4
Barrel + chamber	§ 8.5 + Vol 4 § 4.6 + Vol 7 § 7.7
Magazine (build from scratch — uncommon)	§ 8.6 + Vol 4 § 4.7
Magazine (commercial aftermarket — recommended)	§ 8.6.2 + Vol 11 § 11.3.2
Mk II skeleton T-frame stock	§ 8.7.2 + Vol 7 § 7.8 Step E7
Mk V wooden furniture	§ 8.7.2 + Vol 9 § 9.7 + Vol 7 § 7.12.1

8.10 References (Vol 8)

Vol 4 of this series — Engineering & Reference Data; the dimensional source for every part this volume describes.
Vol 6 of this series — Build Path B; § 6.6.2 (closed-bolt FCG rework) builds on § 8.4.2.
Vol 7 of this series — Build Path C; per-part fabrication procedures cross-referenced section by section.
Vol 9 of this series — Materials & Finishing; case-hardening, surface treatments, wood finishing.
Vol 11 of this series — Live-Fire Operation & Use; § 11.3 (magazine pathology) builds on § 8.6.3; § 11.5 (failure modes) cross-references the “common amateur mistakes” callouts in this volume.
Skennerton, Ian. The Sten Machine Carbine. (Australian-published.) Per-part identification, factory variation, mark-by-mark differences.
Iannamico, Frank. The STEN Submachine Gun. (Moose Lake Publishing.) Engineering detail and reliability context.
Hogg, Ian V., and John Weeks. Military Small Arms of the 20th Century. General reference for Sten-vs-comparable-SMG context.
Professor Parabellum. Practical Scrap Metal Small Arms, Volume III: The DIY STEN Gun. The modern reverse-engineered per-part drawings.
Smith, W.H.B., and Joseph E. Smith. Small Arms of the World. General-encyclopedic reference; useful for cross-variant comparison.