Making One: PSG1-Pattern Rifles from HK91 Parts, the Law, and a Lab Build

9.1 What “making a PSG1” can mean

Nobody in the United States can buy a new PSG1, and its unique parts are scarce German originals (Volume 8). What American builders make instead is a PSG1-pattern rifle: a semi-automatic HK91-type rifle given as many of the PSG1’s features as can be found, bought or made. This volume sets out what that involves, in the order the law and the work require, and it is written for a builder with a mill, a lathe, a press and gunsmithing tools. Its welding steps assume TIG; a shop without a TIG set must add one or have the welds done out of house, and the SA80 dive, Volume 9 sets out those options for a thin pressed receiver. It is a semi-automatic rifle, and a full build path is lawful.

Two limits apply throughout. First, no dimension, sheet thickness, steel specification, heat treatment or torque appears in this volume unless a published source gives it, and for the HK91 receiver almost none do; where a number is missing the text says so. Second, the volume names the parts that make an HK receiver a machine gun only to tell a builder to keep away from them. It gives no conversion detail of any kind.

The hub already has three treatments of the law that this volume builds on and does not repeat: the STEN Gun dive, Volume 6, §6.5 on the § 922(r) parts count; the AK-47 dive, Volume 17, which quotes 27 CFR 478.39 and sets out how the count works; and the Steyr AUG dive, Volume 6 on the frame-or-receiver rule, receiver marking, privately made firearms, kits with select-fire parts and Michigan.

9.2 Read this first: the select-fire parts in every G3 kit

Every G3 parts kit found on sale on 18 October 2026 ships with machine-gun parts.

• Apex Gun Parts’ kit with barrel and trunnion ($1,149): “Select fire trigger pack with S.E.F. marked grip frame”, and “These parts have been taken from fully automatic weapons, and no modifications have been made for semi-automatic.”¹
• HKParts.net’s FMP kit ($159.95, sold out): “Bolt carrier (full auto, complete)” and “SEF full auto metal trigger group, complete”.²
• HKParts.net’s German kit ($397.95, sold out): a “full-auto bolt carrier” and an “SEF trigger group”.³

The trade itself keeps the two kinds apart: Apex lists a “Bolt Carrier, Semi-Auto” for the G3 and HK91 beside its full-automatic carrier groups and select-fire trigger assemblies.⁴

The law that makes this matter is the National Firearms Act’s definition of a machine gun, which includes “any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person” (26 U.S.C. § 5845(b)); the Steyr AUG dive, Volume 6, §6.6, quotes it in full and sets out the same problem for AUG kits.⁵ Michigan’s machine-gun statute, MCL 750.224, makes possession of a machine gun a felony (Volume 8). The conservative course the AUG dive sets out applies here unchanged: remove the select-fire trigger group and the full-automatic carrier from the build before any receiver is acquired, and keep them only with a written ATF classification saying it is lawful to do so. A PSG1-pattern build needs neither part. Its trigger is a semi-automatic pack and its carrier a semi-automatic carrier.

9.3 Which part is the firearm

The federal regulation names the HK receiver. 27 CFR 478.12(a)(4)(ix): “HK machineguns and semiautomatic variants: The receiver is the upper part of the weapon that provides housing for the bolt.”⁶ The stamped upper receiver is the firearm; the grip frame and trigger housing are not. A trunnion, bought loose, is not a firearm; it is a listed part for the § 922(r) count (below). The rule as a whole, upheld by the Supreme Court 7–2 in Bondi v. VanDerStok on 26 March 2025, and the questions of marking and privately made firearms, are in the AUG dive, Volume 6. A search of the Federal Register’s ATF documents from June 2025 to 18 October 2026 found no rule amending 478.12.⁷

A kit’s receiver has been destroyed for import by torch cuts. The remnants that come with some kits (HKParts’ FMP kit lists a “rear receiver section”; Apex’s kit includes the “back half of magazine well and paddle … from a CETME C receiver”) are pieces of destroyed receivers. Whether a given remnant is still a “receiver” turns on whether it was destroyed to the regulation’s standard, which the AUG dive quotes; no ATF statement specific to G3 remnants was found.²¹

9.4 The semi-automatic receiver: shelf, not push pin

What makes an HK receiver a machine-gun receiver is how its trigger housing attaches. The most detailed published account is Thomas Hoel’s in Small Arms Review in April 2000: “The ONLY difference in the receiver of a semi-automatic gun is in its’ method of attaching the trigger housing”; there are “NO internal receiver differences” between a select-fire G3 and the semi-automatic HK91; the front pivot-pin holes have “been determined to be the sole defining feature by BATF Technology Branch”; and on semi-automatic receivers “a U-shaped ledge piece is welded around the normal pivot holes”, or on some Greek-made receivers the holes were punched and then “blocked with a welded in bar”.⁸ The ATF determination itself was not located; the account is a magazine’s report of it.

A builder’s tutorial says the same in plain terms: “On semi-auto guns, the hole for the third pin is blocked, because it would otherwise allow use of a standard machinegun trigger pack.”⁹ The trade builds to it. PTR’s receiver flats are sold with an “HK-spec semi-auto shelf” in their weldment set, and Magpul names its grip module for “HK91 & Semi Shelf HK® Clones”.¹⁰¹¹

Three rules follow for a builder:

1. Build the shelf. A PSG1-pattern receiver is a semi-automatic receiver with the clip-on shelf. Do not drill, punch or reproduce pivot-pin holes.
2. Use a shelf-pattern housing. US-made push-pin trigger housings are sold openly, for registered machine guns and other platforms; HKParts.net lists four-position and three-position push-pin housings that it does not list for the HK91 or PTR.¹² They are the wrong pattern for a shelf receiver.
3. Check any “SEF-style” housing. HKParts.net says the genuine PSG1 trigger pack “requires a metal 0,1 or SEF style housing and a proper PSG1 selector lever”.¹³ An SEF-marked housing from a G3 kit is a select-fire housing; a builder using a PSG1 pack needs a semi-automatic, shelf-pattern housing marked 0 and 1.

PTR’s own catalogue describes its GI rifle as “host-ready for NFA-registered auto sear packs where legally applicable”.¹⁴ That means one thing: a registered drop-in auto sear, made and registered before 19 May 1986, is the only lawful route to automatic fire on a semi-automatic HK receiver, and machine guns still carry the $200 tax after Public Law 119-21.

9.5 Section 922(r) on an HK91-pattern rifle

18 U.S.C. § 922(r) and 27 CFR 478.39 forbid assembling a semi-automatic rifle that could not be imported “using more than 10 of the imported parts listed in paragraph (c)”. The text was confirmed unchanged on eCFR on 18 October 2026.¹⁵ The rule is a ceiling of ten imported listed parts; it sets no quota of US-made parts and does not shrink when fewer than twenty listed parts exist. That point needs making because the hub’s STEN dive, the canonical treatment, has framing sentences in §6.5.1 that state an “at least 10 US-made” version; the AK-47 and AUG dives state the rule correctly, and this volume follows them. (The discrepancy is reported in this dive’s working notes; the STEN volume is not edited here.)

The rule applies to an HK91-pattern rifle because it has a separate pistol grip and a flash hider, two of the features that made the HK91 non-importable in 1989 (Volume 8). Applied to H&K’s own parts list, the count is this dive’s analysis, not an ATF count:

Table 1 — The rule applies to an HK91-pattern rifle because it has a separate pistol grip and a flash hider, two of the features that made the HK91 non-importable in 1989 (Volume 8). Applied to H&K's own parts list, the count is this dive's analysis, not an ATF count

#	Listed part (478.39(c))	HK91 / G3 equivalent	Present?
1	Frames, receivers	Stamped receiver	Yes
2	Barrels	Barrel	Yes
3	Barrel extensions	The trunnion: H&K’s own G3 manual calls it the “barrel extension”	Yes (see below)
4	Mounting blocks (trunnions)	Same part as 3	Count once
5	Muzzle attachments	Flash hider or brake	Yes if fitted
6	Bolts	Bolt head (with control piece)	Yes
7	Bolt carriers	Carrier	Yes
8	Operating rods	None (delayed blowback)	No
9	Gas pistons	None	No
10	Trigger housings	Trigger housing / grip frame	Yes
11	Triggers	Trigger	Yes
12	Hammers	Hammer	Yes
13	Sears	Sear	Yes
14	Disconnectors	The single-fire catch; whether ATF treats it as the disconnector is unsettled	Probably
15	Buttstocks	Stock	Yes
16	Pistol grips	Grip (moulded into the housing on later patterns)	Yes
17	Forearms, handguards	Handguard	Yes
18–20	Magazine body, follower, floor plate	Three parts	Yes, if an imported magazine is in the rifle

That gives 16 to 18 listed parts present, depending on whether the trunnion is counted once and whether the catch is a disconnector. A compliance vendor’s blog gives 17 for “the H&K 91”; that page was seen only as a search excerpt and is not relied on.¹⁶ With a ceiling of ten imported, six to eight listed parts must be US-made, and the AK dive’s point that the count is a property of the rifle as it stands applies: an imported magazine adds three imported parts the moment it is inserted.¹⁷

Three counting traps are specific to this family:

1. The barrel comes in the trunnion. A kit sold “with intact Factory Barrel and Trunnion” carries two imported listed parts in one assembly.¹
2. Imported G3 barrels are on sale again. HKParts.net sells a new Portuguese FMP 17.7-inch barrel, “Portuguese manufactured to HK’s military specifications”, at $89.95.¹⁸ That is consistent with ATF Ruling 2025-1 of June 2025, which rescinded ATF’s 2005 refusal to allow the import of barrels from non-importable military firearms; the link between the ruling and these barrels is this dive’s inference.¹⁹ ATF’s 2026 proposal on dual-use parts is explicit that such parts may be used only “provided assembling such firearm complies with 18 U.S.C. 922(r)”. An imported barrel counts as imported.
3. “US-made” means made in the United States. A part the builder makes from raw stock in his own shop is US-made; the STEN dive’s §6.5.4 sets out the substantial-transformation reading.

A typical count for a kit on a US receiver, as analysis: the kit supplies an imported trunnion, bolt head, carrier, trigger housing, trigger, hammer, sear and catch, stock, grip, handguard and flash hider, about twelve imported listed parts. A US receiver, a US barrel and a US-made magazine take the imported count to about twelve with the magazine excluded; one or two more US parts, from a list that PTR and others make (stock, handguard, grip, flash hider, bolt head, sear), bring it under ten.²⁰ Keep the count in writing with the build record. No HK91-specific compliance sheet from PTR or any other maker was found.

9.6 Three routes

Table 2 — Three routes

Route	What it is	Firearm status	Main work
A. Start from a complete US rifle	Buy a PTR GI or Century CA-3 (assembled by PTR), then fit PSG1-pattern parts	A serialised rifle made by a licensed maker; no privately made firearm questions	Trigger, stock, barrel, mount, reinforcement; keep the count under ten imported parts
B. Kit on a US receiver	A G3 kit (select-fire parts removed) built on a receiver folded from a US flat	The folded receiver is a privately made firearm (see below)	Fold, weld, trunnion, barrel, bolt gap, plus everything in A
C. From scratch	Receiver formed from sheet in the lab, kit or US internals	Privately made firearm	All of B, and the receiver itself without any drawing

Route A’s hosts, from the makers’ own pages on 18 October 2026: PTR Industries of Aynor, South Carolina lists the GI ($1,529–1,610), GIR and GIRK ($1,579–1,669) and A3S and A3SK ($1,645–1,725) rifles; the GI has an 18-inch barrel with a 1:10 twist, a 9–10 lb trigger, a traditional claw mount, 9.5 lb weight and 40.5 inches overall. PTR’s earlier precision models, the MSG91 reviewed in 2016 and a “Super Sniper” reported by Wikipedia, are not in its current catalogue.²¹¹⁴²² Century Arms’ CA-3 ($999.99 MSRP) is “assembled by PTR” from a “military surplus sourced G3”, with an American-made PTR stamped receiver and barrel and a surplus trigger group “modified for compliance”.²³

Route B’s receiver. HKParts.net has sold PTR-made G3 receiver flats, “Made by PTR with original HK tooling using factory specifications”, at $99.95, and a flat “with weldment set” (semi-automatic shelf, reinforcement inserts, rear sight base, push-pin bushings for the stock) at $189.95; both were sold out on 18 October 2026. The listing says “No FFL is required as this is an unfolded flat”, and links folding instructions (the link returned “not found”).¹⁰

That “no FFL” statement needs a warning. 27 CFR 478.12(c) treats a partially complete receiver as a receiver if it “may readily be completed”, and its examples distinguish an AK flat “without laser cuts or indexing” that is “not sold, distributed, or possessed with instructions, jigs, templates, equipment, or tools” (not a receiver) from kits sold with jigs or templates (a receiver).⁶ A stamped G3 flat with its holes punched, sold with folding instructions and with the parts to finish a rifle, sits on the “receiver” side of those examples on their face. No ATF classification of a G3 flat was found, and the regulation voids pre-2022 classifications of partial receivers. This is an open question for ATF, not something this dive resolves (see the questions at the end).

A receiver a person makes for his own use is lawful federally, is a “privately made firearm” (27 CFR 478.11), and carries no federal marking duty for a non-licensee; the AUG dive, Volume 6, §6.3, sets that out with the Michigan position.

9.7 The PSG1 recipe against what can be bought

Table 3 — The PSG1 recipe against what can be bought

PSG1 feature (Volume 3)	Genuine part	What a builder can get, from makers’ and dealers’ pages
Stamped receiver with welded reinforcement	Not available	US flats (sold out); a reinforcement must be made
Heavy 650 mm polygonal barrel	Not available	No PSG1-contour HK91 barrel found from any maker’s page; PTR 18 in US barrel (out of stock); RCM 17.7 in (out of stock); FMP imported 17.7 in
PSG1/MSG90 bolt head	$598, sold out (Midwest Gun Works)	PTR US bolt head $159.95 (Apex)
Control piece (reportedly 40°)	Not sold separately	PTR US control pieces 45°, 55°, 65°, 70°, 90° (Apex); no 40° found
PSG1 trigger pack	$590, sold out (Midwest Gun Works); German pack listed without price, sold out (HKParts)	Reworked HK91 pack, 5–6 lb (HKParts service)
PSG1 grip with palm shelf	$1,985, sold out (Midwest Gun Works)	None found
PSG1 stock	$1,195, sold out (Midwest Gun Works)	Magpul PRS2 HK91 (sold out, not on Magpul’s site); Spuhr R-410 G3 stock (no price shown)
Fixed mount, welded bases	Not available	Claw mounts; welded rail; Spuhr R-402 G3 top rail
Closing aid	Not available	None found
T-slot handguard	Listed, out of stock (HKParts)	Spuhr R-401 G3 forend; US polymer and M-LOK handguards

Sources for the table are the dealers’ and makers’ pages listed in the references.^2441325 The pattern is plain: the parts that make a PSG1 a PSG1 are the parts that cannot be bought new. The trigger, grip, stock, mount and closing aid are scarce German originals or nothing, and the barrel and reinforcement must be found or made.

9.8 The receiver

H&K’s English G3 description gives the only primary description of the receiver’s construction:²⁶

• “On the right and left side of the receiver grooves are impressed to guide the bolt and to seat the back plate.”
• “In front there are two openings to receive barrel extension and barrel, as well as the loading lever housing.”
• “The sight base is welded on the receiver.” (A PSG1 has no sight base; Volume 5.)
• The receiver has “4 recesses for the engaging of telescopic sight mount and clamping claws”.
• “The loading lever housing is inserted in the receiver and spot-welded.”
• The lower rear is “box-type shaped and provided with 2 tubular rivets” to stow the locking pins.

It gives no sheet thickness, steel or heat treatment, and none was found in any vendor listing, tutorial or forum excerpt read. A reviewer of the PTR flat said only that it was “heavier than I was expecting, and thicker”.¹⁰ A builder starting from sheet has no published number to work to; the Bundeswehr’s armourer-level instructions and H&K’s armourer’s manual (Apex sells a translated “G3 Repair Armorer’s Manual” reprint at $15.95) are the likely sources and were not read.²⁷ Measure a genuine receiver or a PTR flat before committing to stock.

Folding a flat is done in a jig, and none is sold by any maker found. Builders’ accounts (forum excerpts only) describe improvised jigs that brace the inside of the receiver while it is rolled, rails straightened afterwards in a jig, and tack welds after forming.²⁸ This is exactly the kind of tooling a shop with a mill can make well, and exactly the kind for which nothing is published.

9.9 Receiver reinforcement

The PSG1’s receiver carries welded reinforcement along its sides (Volume 3), and that is the one PSG1 feature that a builder must make: no part, drawing or dimension for it was found. The German SAR M41 PSG copy carries the same “welded-on reinforcements”, and its tester noted the consequence: “a retractable stock could no longer be fitted”, because the strips sit over the side channels in which a G3’s retractable stock slides.²⁹ Builders’ forum posts mention “US made reinforcement T rails” and two former suppliers of PSG1 rails and trunnions; the website of one now carries unrelated casino advertising and should not be trusted.³⁰

With a mill and TIG welding, in house or hired out, the work is straightforward to describe and specific to get right. The strips stiffen the receiver against flexing between the trunnion and the stock, which is where the PSG1’s fixed mount and heavy barrel need it. Their section, length and the weld pattern that avoids distorting the thin receiver are all unpublished; the only published welding guidance for this receiver is the trunnion procedure below, which warns about burn-through on the thin receiver wall. A builder should treat the reinforcement as a design exercise to be measured from a genuine PSG1 or a good photograph, not copied from a description.

9.10 Trunnion, barrel and bolt gap

The trunnion is welded, not riveted. H&K’s G3 description has the barrel extension received in an opening at the front of the receiver.²⁶ A builder’s tutorial for an HK-pattern rifle describes the trunnion “set flush with the receiver and held with a small tack weld, then welded through to the trunnion and dressed”, using TIG with argon at about 75 A on the trunnion and less over the thin receiver wall, a turned barrel stub to hold the trunnion square, and symmetrical spot-weld holes; it warns that adding filler before the trunnion is hot burns through the receiver.³¹ Every kit supplies a trunnion, and PTR makes a US one.

The barrel is pressed into the trunnion and cross-pinned. H&K’s PSG1 booklet: “The barrel is pressed into the locking piece and pinned.”³² The pin is 5 mm, with a 5.5 mm oversize repair pin; Apex lists both.³³ PTR’s US barrel is “ready to be fitted and drilled for the retaining pin”, with a front journal larger than standard that needs PTR’s own or a modified triple frame (the handguard and front-sight bracket).³⁴ A hydraulic press is needed; forum accounts agree on that, and the force is not published.

The bolt gap is set by the barrel’s position, then trimmed with rollers. The tutorial: the barrel is “pressed out of the old trunnion and pressed into the new one while setting the bolt gap”.⁹ H&K’s G3 figure for the gap is “0.5 – 0.1 mm” (Volume 3). A community wiki gives 0.25 to 0.50 mm “for all HK roller delayed firearms”, without a source; the two minimums differ and both are reported.²⁶³⁵ Rollers are sold in sizes: Apex lists German rollers of 8.00 mm (standard) and 8.02 mm, and Midwest Gun Works an 8.04 mm; the wiki gives 7.96, 7.98, 8.00, 8.02 and 8.04 mm.³⁶³⁵ Marking conventions for oversize rollers disagree between the wiki and the dealer.

The order of work, from the tutorials and H&K’s description: weld the trunnion; press the barrel to the depth that gives a bolt gap inside H&K’s range with standard rollers, leaving room for adjustment either way; cross-drill and pin; then check the gap with a feeler gauge and fine-tune with rollers. Headspace is set by the same barrel position. No H&K headspace figure or gauge procedure was found for the G3 family; a builder should check headspace with gauges for the chamber as he would on any rifle and not assume that a correct bolt gap means a correct headspace.

9.11 The barrel question

The PSG1’s barrel is the part most responsible for its accuracy, and it is the part least available. No maker’s page found offers a PSG1-contour, 650 mm, polygonal HK91 barrel. The options found:

Table 4 — The PSG1's barrel is the part most responsible for its accuracy, and it is the part least available. No maker's page found offers a PSG1-contour, 650 mm, polygonal HK91 barrel. The options found

Barrel	Source	Status 18 October 2026
PTR US barrel, 18 in, fluted chamber, 15×1 thread, in the white	Apex, $199.95	Out of stock34
RCM 17.7 in, 1:10, nitrided, from a “41V50 hammer forged blank”	HKParts.net, $264.99	Out of stock37
FMP (Portugal) 17.7 in, 15×1 thread; imported part	HKParts.net, $89.95	In stock18
PSG-style polygonal barrels from “Rim Country” with “Schneider P5” profiles, about $900	Forum excerpt only; not checked on a maker’s site	Unverified38

Two published data points suggest what is possible. A PSG1 built by Murray Urbach “using all factory parts” sold at auction in 2021 (Volume 8), which shows that factory PSG1 barrels have reached American builders in the past. And the German SAR M41 PSG uses a conventional land-and-groove Lothar Walther barrel, 600 mm, cold-forged and vacuum stress-relieved, with a 22 mm muzzle and a 1:12 twist, and printed five-shot groups of 15 to 25 mm at 100 m in a magazine test (Volume 7).²⁹ A heavy, stress-relieved match barrel of conventional rifling, properly installed, is therefore a demonstrated route to PSG1-class groups on a G3 action. It needs a chamber with the G3’s flutes (Volume 3), which a match-barrel maker would have to cut; no US maker advertising that service was found.

9.12 Bolt group and control piece

Buy the bolt group; do not make it. Use a semi-automatic carrier, never a kit’s full-automatic one (above). PTR makes a US bolt head, sold by Apex at $159.95 as a § 922(r) compliance part.²⁰

The control piece sets the delay ratio (Volume 3), and the trade sells them by angle: Apex lists PTR-made control pieces of 45, 55, 65, 70 and 90 degrees at $37.95 to $59.95.³⁹ A community wiki says the G3’s standard piece is 45 degrees and the PSG1’s and MSG90’s are 40, without citing a source, and a forum account of a PSG1-pattern build mentions “a 40 degree locking piece US” of unnamed make; no US maker of a 40-degree piece was found.³⁵⁴⁰ No published source ties a particular angle to accuracy. A builder who changes the control piece changes the bolt timing and should check function and the bolt gap again.

9.13 Cocking tube

Kits supply the cocking tube, and Apex has listed a US-made replacement. A tutorial gives the critical clearance: with the carrier closed and locked, the gap between the cocking-handle extension and the carrier should be “no more than .020″” (one tutorial) or .022 in (another), “the thickness of a box cutter blade”; too large and the bolt will not unlock properly. The tube is welded in with a thin liner and a copper backing rod.⁹³¹

9.14 Trigger

The PSG1’s trigger is the feature most worth having and hardest to get. Genuine German PSG1 packs were listed sold out at both dealers checked ($590 at Midwest Gun Works).²⁴¹³ HKParts.net’s description of the pack is the practical specification: “3 lb pull”; it “requires a metal 0,1 or SEF style housing and a proper PSG1 selector lever”, and fits “all 90 series firearms with a simple ejector swap”. The Firearm Blog in 2023 described fitting a PSG1 housing to an EBO SAR-8 and noted that the PSG1’s hammer spring needs a U-shaped notch in the safety axle, so that another safety lever cannot be used.¹³⁴¹

Without a PSG1 pack, the alternative found is a reworked HK91 pack: HKParts.net offers to rework “a standard 12lb stock trigger pack” to “a smooth and crisp 5-6lb pull with reduced trigger slack and over-travel”.⁴² PTR’s own GI trigger is 9–10 lb.¹⁴ A Timney HK91 trigger was in beta testing in 2020; no product page for it was found.⁴³ A builder making his own match pack would be making a semi-automatic trigger and is within the law; no drawings of the PSG1’s pack were found, and its internals are unpublished (Volume 3).

9.15 Stock, handguard and mount

Stock. A genuine PSG1 stock ($1,195) was sold out; the Magpul PRS2 for the HK91 ($242 at a dealer) was sold out and is not on Magpul’s own site; Spuhr’s R-410 adjustable G3 stock, in short-buffer and heavy-buffer versions with a length of pull of 320 to 400 mm, shows no price.²⁴⁴⁴²⁵ The stock fits to the receiver by the end cap and two pins, and the end cap carries the recoil spring and buffer (Volume 3), so any stock must match the buffer used.

Handguard. The G3’s handguard hangs from the cocking tube, not the barrel; H&K’s G3 description calls the G3A4’s arrangement a “free floating barrel”.²⁶ A PSG1-pattern build keeps that layout. Spuhr’s R-401 is an aluminium G3 forend “matching the outline and weight of the original HK plastic forend” with Spuhr’s interface for accessories; Apex lists US polymer HK91 handguards ($29.95, a § 922(r) part) and others list M-LOK and KeyMod handguards.²⁵²⁰ None is a PSG1 T-slot handguard; a builder who wants the PSG1’s tripod and sling interface will add an Anschütz-type rail.

Mount. No reproduction of the PSG1’s fixed mount was found. The routes are a claw mount on the receiver’s four recesses (standard and removable), a welded rail (the German copy uses a welded 26 cm Picatinny rail), or Spuhr’s R-402 G3 top rail.²⁹²⁵ A forum account attributes 3 to 4 inch groups at 100 yards on a PTR to a flexing low-cost claw mount; that is a forum report, but it points at the PSG1’s own reason for a fixed mount.⁴⁵

9.16 The closing aid

No published part, drawing or method for fitting a PSG1-style closing aid to an HK91-pattern rifle was found, and the PSG1’s own mechanism is unpublished (Volume 3). What is known is that it sits on the right of the receiver behind the ejection port, that genuine PSG1 carriers are “serrated for the closure device”, and that H&K’s purpose for it is to seat the bolt fully after a quiet, hand-eased closing.³²⁴⁶ For a lab it is a design project: a spring-returned plunger or lever on the receiver engaging serrations cut in a semi-automatic carrier, in the manner of an AR-15 forward assist, which is the comparison the secondary sources draw. It is a semi-automatic feature and raises no legal question. Its geometry, travel and the depth of the carrier serrations have to be worked out from a genuine PSG1 or from first principles; this dive gives none.

9.17 Finish

The published tutorial’s builder bead-blasted the rifle and used a satin black spray-on finish; an HKParts reviewer suggested parkerising.³¹¹⁰ The German PSG1 copy’s receiver and trigger housing are described as manganese phosphated; H&K’s own finish specification for the PSG1 was not found.²⁹ The hub’s finishing methods are in the STEN dive’s materials volume.

9.18 Proving the build

Before the first live round:

1. Function check in the Bundeswehr regulation’s manner, adapted to a semi-automatic (Volume 4): with dummy rounds, feed from the magazine; trigger blocked at safe; hammer falls at fire; cycle by hand and confirm extraction, ejection and a reset click when the trigger is released; confirm the bolt locks fully.
2. Bolt gap inside H&K’s range, measured with feeler gauges.
3. Headspace with gauges.
4. Control-piece and roller check: rollers turn freely, no marks on the control piece’s faces (the Bundeswehr’s inspection criteria, Volume 4).
5. The § 922(r) count, written down and kept with the receiver’s record.
6. Live fire with a single round loaded, then two, watching ejection: right and forward is normal; square or rearward is the regulation’s sign of a problem (Volume 4).

9.19 Cost

No published cost for a PSG1-pattern build was found from a maker. Forum posts (excerpts only) estimate “about $10K minimum” for a rifle on a non-German host with mostly non-German parts, and some advise buying a factory PSG1 instead.⁴⁷ The parts table above shows why: the genuine PSG1 trigger, grip, stock and bolt head alone were listed at about $4,400 in total before they sold out, and the barrel and reinforcement are labour or unavailable.

9.20 Michigan

The hub’s Michigan coverage applies (Volume 8 and the AUG dive, Volume 6, §6.10). Three points are specific to building:

• Buying a receiver or a complete host rifle is buying a firearm that is not a pistol, under MCL 28.422 as amended by 2023 PA 19, with its dealer exemption.
• No state serialisation of privately made firearms was found in the AUG dive’s search of the Michigan statutes as of September 2026; that search was not repeated for this dive, so legislation enacted since then is unchecked.
• Marks of identity. MCL 750.230 makes it a felony to “wilfully alter, remove, or obliterate” a firearm’s maker’s name, model or number. A builder refinishing or reinforcing a used receiver leaves its markings alone.
• Machine guns. MCL 750.224, quoted in the AUG dive. Nothing in it is a parts clause, but the federal combination-of-parts rule applies in Michigan as everywhere.

9.21 Questions only ATF, a lawyer or a maker can answer

Table 5 — Questions only ATF, a lawyer or a maker can answer

Question	Who can answer
Is a stamped, punched G3 receiver flat sold with folding instructions a “frame or receiver” under 27 CFR 478.12(c)?	ATF, by classification letter
Is a particular torch-cut G3 receiver remnant still a receiver?	ATF
Does ATF count the trunnion as a “barrel extension”, a “mounting block”, or both?	ATF
Is the HK single-fire catch a “disconnector” for § 922(r)?	ATF
Has Michigan enacted serialisation of privately made firearms since September 2026?	A Michigan lawyer, or the Michigan State Police
What is the HK91 receiver’s sheet thickness and steel?	H&K’s armourer’s documentation, or measurement
Will a match-barrel maker cut G3 chamber flutes in a PSG1-contour barrel?	Individual barrel makers

9.22 What could not be established

• The HK91 receiver’s sheet thickness, steel and heat treatment.
• Any drawing of the receiver, trunnion, PSG1 reinforcement, PSG1 trigger or closing aid.
• A current maker of a PSG1-contour barrel, a 40-degree control piece, a PSG1 stock or grip reproduction, or a fixed PSG1 mount.
• An HK91-specific § 922(r) compliance sheet from any maker.
• ATF’s classification of G3 flats and remnants, and the ATF letter behind the pivot-pin rule.
• The headspace specification for the G3 family from H&K.

References

Footnotes

1. Apex Gun Parts, “Original FMP made H&K G3 Parts Kit with intact Factory Barrel and Trunnion, Select Fire Grip (S.E.F.), Black Polymer Stock 7.62x51 NATO Good”, SKU HNKKT-22022-OF, $1,149, https://www.apexgunparts.com/hk-g3-parts-kit-7-62x51-nato-good.html, consulted 2026-10-18. ↩ ↩² ↩³

2. HKParts.net, “HK G3 Parts Kit - FMP”, HKP-22531, $159.95, sold out, https://hkparts.net/parts-kits/hk-g3-parts-kit-fmp/, consulted 2026-10-18. ↩ ↩²

3. HKParts.net, “HK G3 Parts Kit”, HKP-00827, $397.95, sold out, https://hkparts.net/hk-rifle-smg-parts/hk-g3-parts-kit/, consulted 2026-10-18. ↩

4. Apex Gun Parts listings for G3/HK91 semi-automatic and full-automatic bolt carriers, select-fire trigger assemblies and PSG1-related parts, https://www.apexgunparts.com/, consulted 2026-10-18. ↩ ↩²

5. Hub: Steyr AUG, Volume 6, §§6.2–6.6 and 6.10, quoting 26 U.S.C. § 5845(b), 27 CFR 478.11, 478.12 and 478.92, and the Michigan statutes. ↩

6. 27 CFR 478.12, eCFR, current text read 2026-10-18, paragraphs (a)(4)(ix), (c) and (f)(2), https://www.ecfr.gov/current/title-27/section-478.12. ↩ ↩²

7. Federal Register API, ATF documents published 1 June 2025 to 18 October 2026 (48 documents), searched 2026-10-18. 91 FR 24348 (6 May 2026) amended only the bump-stock sentences of the machine-gun definitions. ↩

8. Thomas Hoel, “The Mechanics of the Heckler & Koch NFA Conversions: Part I”, Small Arms Review vol. 3 no. 7, April 2000, https://smallarmsreview.com/the-mechanics-of-the-heckler-koch-nfa-conversions-part-i-2/, consulted 2026-10-18. Only the passages on receiver identification are used. ↩

9. Arizona Response Systems, “HKHK Tutorial: Build an SW3 HK Clone”, 20 September 2016, updated 9 April 2020, consulted 2026-10-18. ↩ ↩² ↩³

10. HKParts.net, “HK91, G3 Receiver Flat - US Made”, HKP-02320, $99.95, sold out, https://hkparts.net/922r-compliant-parts/hk91-g3-receiver-flat-us-made/, and “HK91, G3, CETME Receiver Flat with Weldment Set”, HKP-02086, $189.95, sold out, https://hkparts.net/hk-rifle-smg-parts/hk91-g3-cetme-receiver-flat-with-weldment-set/, consulted 2026-10-18 (including a customer review). ↩ ↩² ↩³ ↩⁴

11. Apex Gun Parts, Magpul “HK94 / HK93 / HK91 & Semi Shelf HK® Clones SL Grip Module”, out of stock, consulted 2026-10-18. ↩

12. HKParts.net, US-made four-position and Navy three-position push-pin trigger housings, $264.95, sold out, compatibility as listed, consulted 2026-10-18. ↩

13. HKParts.net, “HK PSG1 Trigger Pack Assembly - German”, HKP-00907, MPN 224696, https://hkparts.net/hk-rifle-smg-parts/hk-psg1-trigger-pack-assembly-german/, consulted 2026-10-18. ↩ ↩² ↩³ ↩⁴

14. PTR Industries, “PTR GI Rifle”, https://www.ptr-us.com/product/gi/, consulted 2026-10-18. ↩ ↩² ↩³

15. 27 CFR 478.39, eCFR renderer, current text read 2026-10-18, https://www.ecfr.gov/current/title-27/section-478.39. ↩

16. Orchid Advisors, “How to Comply with 18 USC 922(r)”, seen as a search excerpt only, 2026-10-18; not relied on. ↩

17. Hub: AK-47, Volume 17, “The Count Is a Property of the Rifle as It Stands”. ↩

18. HKParts.net, “HK91, G3 Barrel - FMP - 17.7” - Threaded 15x1”, HKP-22532, $89.95, in stock, consulted 2026-10-18. ↩ ↩²

19. ATF, “Importing Dual-Use Frames, Receivers, or Barrels”, proposed rule, 91 FR 25192 (8 May 2026), describing ATF Ruling 2025-1 (23 June 2025), https://www.federalregister.gov/documents/2026/05/08/2026-09163/importing-dual-use-frames-receivers-or-barrels, consulted 2026-10-18. ↩

20. Apex Gun Parts, § 922(r) compliance parts for the G3/HK91: “PTR Manufactured G3/HK91 Bolt Head, No Extractor Assembly … US Made 922(r) Compliant Part” $159.95; US semi-automatic sear $14.95; US polymer handguard $29.95; PTR flash hider and buttstock (out of stock), consulted 2026-10-18. ↩ ↩² ↩³

21. PTR Industries, firearms catalogue, https://www.ptr-us.com/products/firearms/, and frequently asked questions, https://www.ptr-us.com/support/frequently-asked-questions/, consulted 2026-10-18. ↩

22. Michael O. Humphries, “Review: PTR Industries MSG 91 Rifle”, Shooting Illustrated, 27 December 2016, https://www.shootingillustrated.com/content/review-ptr-industries-msg-91-rifle/, consulted 2026-10-18. ↩

23. Century Arms, “CA-3”, SKU RI5601-X, MSRP $999.99, https://www.centuryarms.com/ca-3.html, consulted 2026-10-18. ↩

24. Midwest Gun Works: “Heckler and Koch PSG1 Complete Buttstock” (224721) $1,195.00; “Heckler and Koch PSG1 / HK91 Trigger Mechanism” (221020) $590.00; PSG1 grip assembly $1,985; “PSG1/MSG90 Complete Bolthead” (225656) $598.00; all sold out, https://www.midwestgunworks.com/, consulted 2026-10-18. ↩ ↩² ↩³

25. Spuhr i Dalby AB, “H&K G3 Upgrades” (R-401, R-402, R-410), https://spuhr.se/pages/h-k-g3-upgrades, consulted 2026-10-18. ↩ ↩² ↩³ ↩⁴

26. Heckler & Koch, Technical Description of the Automatic Rifle HK G3, Cal. 7.62 mm NATO (English; licensed reprint, copyright Heckler & Koch Inc.), receiver, barrel and handguard descriptions and technical data. https://archive.org/download/MManuals/HkG3AutomaticRifleCal7.62mmNato.pdf, consulted 2026-10-18. ↩ ↩² ↩³ ↩⁴

27. Apex Gun Parts, “G3 Repair Armorer’s Manual, Translated From Original”, $15.95, consulted 2026-10-18 (not read). ↩

28. Sniper’s Hide and Reddit r/ptr91 threads on folding G3 flats, seen as search excerpts only, 2026-10-18. Forum reports. ↩

29. Franz Keck, “Präzision mit Geschichte”, Deutsches Waffen-Journal, January 2023, pp. 20–25, https://www.schwabenarmsgmbh.de/index_htm_files/Testbericht%20DWJ%20Ausgabe%2001-2023%20SAR%20M41%20PSG.pdf, consulted 2026-10-18. Translated for this dive. ↩ ↩² ↩³ ↩⁴

30. hkpro.com threads “PSG1 Clone Build” (2012) and “Starting a PSG1 Clone”, seen as search excerpts only (the site redirects automated readers to a paid gateway), 2026-10-18. Forum reports. ↩

31. Arizona Response Systems, “Build a JLD PTR-91 HK Clone” tutorial, 2016, updated 2020, consulted 2026-10-18. ↩ ↩² ↩³

32. Heckler & Koch GmbH, Kurzbeschreibung des Präzisionsschützengewehres PSG 1 (Oberndorf, n.d.), printed pp. 3, 6. https://archive.org/download/MManuals/HkPsg1.pdf, consulted 2026-10-18. ↩ ↩²

33. Apex Gun Parts, “H&K G3/HK33/MP5 Barrel Pin, 5mm Standard” ($9.95–10.95) and “HK Barrel Retaining Pin, Oversize, 5.5mm” ($4.95), consulted 2026-10-18. ↩

34. Apex Gun Parts, “HK G3 / HK 91 / PTR 91 Rifle Barrel, Fluted Chamber, 15x1 Muzzle Thread, 7.62X51, 922(r) Compliance Part NEW, US Made by PTR”, SKU BBL-17803, $199.95, out of stock, https://www.apexgunparts.com/g3-hk91-barrel-fluted-chamber-15x1-muzzle-thread-7-62x51-922-r-compliance-part-new.html, consulted 2026-10-18. ↩ ↩²

35. “HK Bolt Gap” and locking-piece table, Thinline Weapons wiki, https://thinlineweapons.com/wiki/index.php/HK_Bolt_Gap, edited 21 February 2026, consulted 2026-10-18. No sources cited. ↩ ↩² ↩³

36. Apex Gun Parts, German H&K locking rollers +0 (8.00 mm) and +2 (8.02 mm), and roller sets, consulted 2026-10-18; Midwest Gun Works, “G3 / HK91 (8.04mm) +4 Locking Roller”, https://www.midwestgunworks.com/page/mgwi/prod/200448, consulted 2026-10-18. ↩

37. HKParts.net, “HK91, G3 Barrel - 7.62x51 - 17.7” - Threaded 5/8x24”, HKP-21838, RCM, $264.99, out of stock, consulted 2026-10-18. ↩

38. Sniper’s Hide thread “Building a clone HK PSG-1”, seen as a search excerpt only, 2026-10-18. Forum report; not checked with the makers named. ↩

39. Apex Gun Parts, PTR-made G3/HK91 locking pieces (control pieces) of 45°, 55°, 65°, 70° and 90°, $37.95–59.95, consulted 2026-10-18. ↩

40. hkpro.com “PSG1 Clone Build” (2012), seen as a search excerpt only, 2026-10-18. Forum report. ↩

41. Nicholas C., review of a PSG1 trigger housing fitted to an EBO SAR-8, The Firearm Blog, 10 February 2023, consulted 2026-10-18 (as recorded in this dive’s research notes). ↩

42. HKParts.net, “HK 91/G3 Overview” (blog), https://hkparts.net/blog/hk-91g3-overview/, consulted 2026-10-18. ↩

43. The Firearm Blog, report of Timney’s HK91/PTR-91 trigger beta test, 25 August 2020, as recorded in this dive’s research notes; no Timney product page found, 2026-10-18. ↩

44. Midwest Gun Works, “Magpul PRS2 HK 91 / G3 Precision-Adjustable Stock”, MAG340-BLK, $242.00, sold out, consulted 2026-10-18; not found on magpul.com, 2026-10-18. ↩

45. Sniper’s Hide thread on PTR-91 accuracy, seen as a search excerpt only, 2026-10-18. Forum report. ↩

46. Rock Island Auction Company, 25 August 2024, lot 3589, https://www.rockislandauction.com/detail/4092/3589/heckler-koch-psg1-semiautomatic-sniper-rifle-with-scope, consulted 2026-10-18. ↩

47. hkpro.com threads “PSG-1 Clones” and “PSG1 Clone Advice” (2025), seen as search excerpts only, 2026-10-18. Forum reports. ↩