Build Path C — From Scratch in 9×19 (Lab Step-by-Step)

The centerpiece lab build — every part fabricated in-house, op sequence per part

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

This is the centerpiece volume of the series — the step-by-step lab manual Jeff explicitly asked for (“step by step guide of how we will make ours”). It takes the engineering archive cataloged in Vol 4 and turns it into an op sequence: per-part fabrication, fixtures and jigs, in-process inspection, surface finishing, assembly, and test-fire. The volume is built from the UK MoS drawings (cited by SA-prefix number), the Professor Parabellum Vol III reverse-engineered modern drawing set, and Vol 4’s dimensioned reference data — none of which it reproduces directly. Vol 4 has the dimensions; Vol 7 has the procedure.

Two important caveats up front:

Lab-catalog dependence. Some procedure steps below cite specific machine setups (CNC mill spindle RPM, lathe feeds, weld settings). These are marked (lab-catalog confirmation needed) where Jeff’s specific machine model + travels + spindle envelope hasn’t been captured in _shared/lab_capability.md yet. The procedure is correct at the principle level; the specific parameters need confirmation against the actual machine once the catalog is populated. Until then, the parameter callouts are conservative defaults from machinist references (Machinery’s Handbook, the South Bend Lathe Works manual, the Sheet Metal Forming Handbook).
Legal posture. This volume documents a semi-auto, closed-bolt, 9×19 lab build for personal use under the federal “manufacture for personal use” exemption (no federal serialization requirement; some states require US-made 80% receivers be FFL-shipped to the builder; Vol 10 owns the per-state rules). The closed-bolt FCG approach is mandatory for any post-1982 new semi-auto SMG build (Vol 4 § 4.4.2 + Vol 10 § 10.4); do not attempt to fabricate an open-bolt FCG — pre-1982 open-bolt semi-auto STENs are grandfathered, but new builds must be closed-bolt. Vol 10 has the full posture; consult it before any cut metal.

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

7.1.1 Scope of this volume

Vol 7 documents a Mk II receiver, Mk II skeleton T-frame stock, closed-bolt semi-auto-only FCG, 9×19 chamber, pre-rifled barrel-blank with in-lab chambering, manganese-phosphate parkerized finish lab build — the canonical Vol 7 target. § 7.12 covers two stretch options (Mk V wood-furniture conversion, NFA Class III pre-86-registered open-bolt full-auto via Path A combined with Path C parts fabrication for non-receiver components).

7.1.2 Prerequisites

Lab capability (lab-catalog confirmation needed): CNC mill with ≥250 mm travels in X and Y, ≥150 mm Z; lathe with ≥250 mm between centers; sheet-metal roller capable of forming 1.5 mm cold-rolled steel into a ~38 mm OD cylinder; TIG welder (200 A class, capable of welding 1.5 mm mild steel); drill press with depth-stop and X-Y table; 9×19 chamber reamer (SAAMI spec); headspace gauges (GO + NO-GO, 9×19); parkerizing tank (10–15 liter capacity) or hot-blue tank as an alternative; 3D printer for fixture/jig prototyping.
Materials in hand (Vol 4 § 4.9 has the full table): 1018 CR sheet at 0.060″ (1.5 mm), 4140 round stock at ~32 mm OD for the bolt + ~16 mm for FCG parts, 6061-T6 bar stock for the FCG housing (optional alternative to stamped sheet), 1018 round bar at 9.5 mm for the stock, a 9×19 pre-rifled barrel blank from a US barrel-blank vendor, FCG springs (music wire ASTM A228), recoil spring, two Sterling-pattern aftermarket magazines (Bear Arms or ASP).
Skill prerequisites: bench gunsmithing competence; TIG welding on 1.5 mm mild steel (the receiver-tube seam is the only structural weld; everything else is fitting); CNC mill operation with 3-axis programming; lathe operation for bolt + barrel work; chamber reaming with headspace verification (the only operation where wrong-and-not-caught can wreck the gun or hurt the shooter — verify against a GO gauge, not just feel).
Legal posture verified: state-of-residence permits the manufacture of a non-NFA semi-auto SMG-pattern firearm for personal use (most states permit; Vol 10 has the per-state map); state-of-residence SBR rules understood (the 7.75″ barrel is a federal SBR by definition; extending to ≥16″ avoids SBR posture or accept SBR + Form 1 + tax stamp).

7.1.3 Expected outcome

A functional Mk II-pattern closed-bolt semi-auto 9×19 carbine, ~3.2 kg, ~30″ long with stock attached, capable of feeding modern Sterling-pattern magazines and firing standard 9×19 NATO ball at the range. Cosmetic finish per the period-correct manganese-phosphate parkerized spec (Vol 9 has finishing alternatives). Per-build cost (materials only, not amortizing lab tooling): approximately $300–800 depending on barrel-blank vendor + parkerizing supplies + chamber reamer cost (the reamer is reusable across multiple builds; the per-first-build cost is $200 above the per-second-build cost).

Time-on-task: approximately 40–80 lab hours for a first build, distributed across the five work packages. About two weekends of focused shop time for a builder familiar with each operation. The 1941 Fazakerley production rate of one Mk II every 12 seconds is not a realistic comparison — they had purpose-built jigs and a skilled production-line workforce; Jeff has a more-versatile lab but is doing every operation from scratch.

7.2 The five work packages — overview of the build

Vol 7 organizes the build into five work packages, each one a coherent set of operations that can be completed in a single shop session (or one over a weekend, depending on the package). The packages are independent enough that you can sequence them across multiple sessions, with assembly + fitting at the end:

Package	What it makes	Estimated time	Section
A	Receiver tube (the main pressure-bearing structural element)	8–16 hrs	§ 7.4
B	Trigger group housing + closed-bolt FCG (the legal-pivot element)	12–24 hrs	§ 7.5
C	Bolt + firing pin (the operating element)	8–12 hrs	§ 7.6
D	Barrel (chamber in-lab; rifling pre-purchased)	4–8 hrs	§ 7.7
E	Assembly, fitting, surface finishing	8–20 hrs	§ 7.8

The recoil spring, FCG springs, sights, stock, cocking handle knob, extractor, muzzle bushing, and rear cap are all fabricated as part of one of the five packages (mostly Package B or E). The magazine is not fabricated — use commercial Sterling-pattern aftermarket (Vol 4 § 4.7.2 + Vol 11 § 11.3).

A logical sequence:

Package A first (receiver tube — establishes the gun’s central reference axis).
Package C second (bolt — must fit the receiver-tube ID).
Package D third (barrel — must fit the receiver-tube chamber boss + the bolt face headspace).
Package B fourth (FCG — must engage the bolt’s sear surface and align with the magazine well).
Package E last (assembly, fitting, finishing).

This sequence preserves the ability to dimensionally calibrate each subsequent part against the previously-fabricated one. Skip a step and you risk having to re-make a part to match a downstream dimension that crept.

7.3 Materials, tooling, and shop setup

Detailed materials specifications are in Vol 4 § 4.9. The tooling and shop setup specific to Vol 7 (above and beyond the lab catalog):

7.3.1 Fixtures and jigs

The build benefits from several purpose-built fixtures, each of which can be prototyped on Jeff’s 3D printer and finalized in metal once the geometry is confirmed:

Receiver-tube rolling mandrel — a steel cylinder of ~36 mm OD that the sheet wraps around during the roll; gives the wrapped tube a consistent ID without the seam bowing inward. ~$50 in 4140 round stock + lathe time, or a 3D-printed PLA mandrel for prototyping the geometry.
Receiver-tube welding fixture — a clamp arrangement that holds the wrapped tube circular during the longitudinal TIG-weld and provides a copper backing strip to control the seam bead profile. Vol-7-specific; the Professor Parabellum Vol III drawings include a buildable version.
CNC mill receiver-tube fixture — a V-block that registers the welded tube on the mill bed and holds it secure for cutting the magazine-well slot, ejection port, cocking-handle slot, and trigger-group attachment holes. A printed PLA fixture works for prototyping; an aluminum fixture works for production.
Bolt-turning steady rest (lab-catalog confirmation needed if the lathe has a built-in steady rest) — needed to support the bolt’s long thin section during the turning operation.
Chamber-reaming fixture — a lathe-mounted holder for the barrel that allows the reamer to enter coaxially with the bore. A standard chamber-reaming setup; lathe-shop-grade off-the-shelf.
Headspace-gauging fixture — barrel held in the receiver chamber boss + GO/NO-GO gauges + bolt-face position-test arrangement. Critical for verifying the chamber-to-bolt headspace before assembly.

7.3.2 Inspection equipment

Calipers and micrometers to ±0.01 mm for the bolt, ±0.05 mm for the receiver tube, ±0.02 mm for the chamber.
GO + NO-GO 9×19 headspace gauges — non-negotiable for chamber verification.
Bore mirror or borescope for verifying chamber finish and bore condition post-rifling-blank-finish.
Magnetic squares and machinist parallels for fixture setup.
Surface plate or known-flat reference for the trigger group housing assembly verification.

7.3.3 Welding setup

The receiver-tube longitudinal seam is the only structural weld. Setup:

TIG (200 A class) with 2% thoriated tungsten at ~2.4 mm dia, sharpened to a 2:1 cone.
ER70S-2 or ER70S-6 filler at 1.6 mm dia (or seam-melt-only with no filler — practical for 1.5 mm sheet with proper joint prep).
Argon shielding at ~15 cfh.
Copper backing strip behind the joint to control bead profile and prevent burn-through.
Pre-weld: degrease + wire-brush; verify joint gap ≤0.2 mm.
Post-weld: stress-relieve at ~620 °C for 30 min if available (lab-catalog confirmation needed); otherwise air-cool slowly + verify dimensional stability on the OD mandrel before further machining.

7.4 Work Package A — The receiver tube (step-by-step)

Estimated time: 8–16 hours for a first build (mostly setup + fixture-build); subsequent receivers come in at 4–6 hrs.

Step A1 — Cut the sheet blank

Cut a rectangular blank from 1.5 mm 1018 CR sheet:

Width (along the eventual receiver-tube length): ~533 mm (21.0″). Add 5 mm for end-trim allowance → cut at 538 mm.
Length (around the eventual circumference): π × 38 mm (the target OD) = ~119.4 mm. Add 5 mm for the seam-grind allowance and 2 mm for the wrap-spring-back compensation → cut at 126 mm.

Cutting method:

Preferred: 100 W laser cutter or CNC plasma — gives a clean square edge that requires no further prep.
Acceptable: Bandsaw + edge dressing on the disc sander to ±0.2 mm.
Avoid: hand-shear — leaves a slight bend in the cut edge that throws off the wrap.

Mark the interior-side reference line with a sharpie or scribe — this is the surface that becomes the receiver-tube interior; the mark identifies which way to wrap. Mark the seam-side reference line for the longitudinal seam orientation (top-left per the MoS spec; Vol 4 § 4.3.1).

Step A2 — Edge prep for the seam

The longitudinal seam must butt-weld cleanly. Both seam edges need:

Perpendicular to the sheet face (a slight bevel here is fine — but parallelism between the two edges matters).
Smooth + clean — no burrs, no oxide, no machining swarf. Wire-brush + degrease both edges.
Joint gap ≤0.2 mm when the wrapped tube is clamped on the mandrel (verify by feeler gauge before tacking).

Step A3 — Roll the wrap

Set up the sheet-metal roller (or three-roll bender):

Verify the roller’s centerline against the mandrel — the wrap will form around this axis.
Insert the sheet with the interior-side facing the mandrel.
Start with the rolls open (large clearance) and progressively close to the wrap-pressure setting.
Roll the sheet through the rollers until the wrap forms a cylinder of approximately the target diameter (~38 mm OD).
Spring-back from cold-rolled 1018 will be ~2–4 mm of un-recovered curl; the roll-on-mandrel compensates.
Verify on the mandrel: the wrapped tube should slip onto the 36 mm mandrel with ~1 mm radial clearance.
Clamp the wrap to the mandrel with hose clamps or a custom welding fixture (Vol 7 § 7.3.1).

Step A4 — Tack-weld the seam

With the wrap clamped to the mandrel:

Verify seam alignment + gap (≤0.2 mm).
Tack-weld at 4–6 locations along the seam length, starting from the center and working outward. Each tack is a few-second TIG arc, no filler if the joint is tight; with filler if the joint has any visible gap.
After tacking, let the mandrel cool (or remove and let the wrapped tube cool) before the full seam-weld. The tacks should hold geometry through the full-weld thermal cycle.
Verify dimensional stability post-tack: the OD around the tack-weld zone should still be within ±0.5 mm of the target 38 mm.

Step A5 — Full longitudinal seam weld

Set up the TIG with the parameters in § 7.3.3:

Backing strip + copper backing — slide a copper bar inside the wrapped tube along the seam-line to support the underside of the weld pool.
Welding direction — typically rear-to-front (away from the operator), running a continuous bead from one end of the seam to the other.
Speed and amperage (lab-catalog confirmation needed) — approximately 80–100 A with a steady manual travel rate; the goal is a clean fusion bead with no burn-through and minimal heat distortion.
Post-weld: allow the assembly to cool slowly. Do not water-quench (1018 is mild steel; quenching is unnecessary and risks distortion).
Inspect: the bead should be continuous, with no porosity visible, no skips, and no excessive bead-height (which means weld profile to flush or even slight concave on the OD; ID-side bead-height controlled by the copper backing).

Step A6 — Verify the tube post-weld

Pull the wrapped tube off the mandrel:

OD check: caliper around the circumference at 5 locations (rear, middle, front, and two intermediate). Target ~38 mm ± 0.5 mm; reject if any reading is >39 mm or <37 mm.
ID check: micrometer the ID at the same locations. Target ~35 mm; the bolt slides into ~32 mm OD so 35 mm ID gives ~1.5 mm radial clearance to the receiver wall.
Length check: caliper end-to-end. Target 533 mm ± 2 mm.
Straightness check: roll the tube on a flat surface and look for rocking. A small amount of curvature is acceptable; >2 mm runout over the full length warrants a stress-relief cycle.

Step A7 — Stress relief (lab-catalog confirmation needed)

If lab capability supports a sub-critical anneal:

Heat the welded tube to ~620 °C for 30 min in an oven or with an oxyacetylene torch.
Allow to cool slowly in the oven or under a sand bed.
Re-verify dimensions in Step A6.

If lab capability does not support sub-critical anneal, the as-welded tube is probably fine for a 9×19 pressure regime, but the procedure is more-conservative with the stress-relief step. Vol 11’s break-in procedure provides additional verification.

Step A8 — Machine the cutouts

Set up the receiver tube in the CNC mill V-block fixture:

Magazine-well attachment slot on the left side — rectangular cutout, 60 mm × 30 mm, centered ~220 mm forward of the rear end. CNC end mill, climb-cut, multiple passes; finish-pass at +0.05 mm and dress-fit later.
Ejection port on the right side — rectangular cutout, 55 mm × 24 mm, opposite the magazine well at the same fore-aft location.
Cocking-handle slot on the right side, upper rear — longitudinal slot 110 mm long × 8 mm wide, starting ~50 mm aft of the ejection port and running rearward.
Trigger group housing attachment holes on the underside — two M5 (or 1/4″-28) clearance holes for the housing-retaining bolts. Located approximately at the magazine-well fore-aft position.

Step A9 — Receiver-tube fitting marks

After all cutouts are machined:

Sand the cutout edges with a hand file to remove sharp edges and machine swarf.
Mark the seam location with a permanent marker on the OD (for assembly orientation reference).
Mark the rear of the tube with a permanent marker (so the front end is unambiguous during subsequent ops).

Receiver tube is now ready for the muzzle bushing (Package E, Step E2) and rear cap (Package E, Step E3) attachment. Set the tube aside in a clean storage location.

7.5 Work Package B — The trigger group housing + closed-bolt FCG (step-by-step)

Estimated time: 12–24 hours for a first build (this is the largest single work package; the closed-bolt FCG is mechanically different from the WWII open-bolt design and requires careful fitting).

Step B1 — Decide the housing material

Two paths (Vol 4 § 4.5):

Stamped sheet: 16-gauge 1018 mild steel, folded and welded — period-correct for a UK Mk II. Recommended for builders who want the original Mk II look.
CNC-milled bar: 6061-T6 aluminum or 1045 steel, machined from solid — period-correct for a Long Branch Mk II and aesthetically cleaner. Recommended for builders who want the cleanest cosmetic result and have CNC capacity.

Vol 7’s canonical path is the CNC-milled 6061-T6 aluminum housing — it’s faster than the stamped sheet by ~6 hours and gives a dimensionally tighter result.

Step B2 — CNC the housing (6061-T6 path)

Fixture: 6061-T6 bar stock approximately 175 mm × 50 mm × 38 mm on the CNC mill bed.

Operations (in roughly this order):

Top face surface and squaring — make sure the bar stock is square to the mill’s coordinate system.
Rough out the housing outline — leave 1 mm finish allowance.
Magazine-well slot through the housing thickness — rectangular pocket 60 mm × 30 mm, matching the receiver-tube cutout from Step A8.
Trigger guard pocket — rectangular slot on the bottom-rear for the trigger blade and guard.
Internal mechanism pockets — sear pocket, disconnector pocket, magazine-catch pocket, fire-mode-selector pocket (omitted for the semi-auto-only build), trigger pivot, sear pivot, disconnector pivot.
Closed-bolt FCG features (new vs WWII open-bolt design) — firing-pin striker pocket, striker pivot, striker spring retention. The Professor Parabellum Vol III drawings include the closed-bolt FCG layout — use those drawings as the dimensional source for these features.
Receiver-attachment bolt holes — two M5 (or 1/4″-28) holes through the top of the housing aligned to the receiver-tube underside holes (verify by clamp-up to the receiver before drilling).
Pistol grip stub — left as a stub on the Mk II; on a Mk V stretch build, this becomes a full-length pistol grip with grip-panel attachment.
Finish surface — light surface dressing on the visible faces.

Inspection: caliper-check the receiver-attachment hole spacing against the receiver-tube hole spacing (Step A8). If they don’t match within 0.5 mm, redrill the receiver tube to match the housing rather than redoing the housing (the housing is the more-expensive part).

Step B3 — Fabricate the closed-bolt FCG internal parts

The internal mechanism parts (trigger, sear, disconnector, magazine catch, firing-pin striker, springs) are fabricated from 4140 round/bar stock on the CNC mill or lathe.

Per-part details (each is a separate small op):

Trigger — flat plate with pivot hole + sear-engagement surface. ~3 mm thick, machined on CNC mill from 4140 bar.
Sear — flat plate with pivot hole + trigger-engagement notch + bolt-engagement surface. ~4 mm thick, machined from 4140; case-hardened on the engagement surface to Rc 50–55.
Disconnector — flat plate with pivot hole + sear-reset surface. ~3 mm thick from 4140.
Firing-pin striker (new for closed-bolt) — small machined part that pivots and strikes the firing pin from the rear. Per Professor Parabellum Vol III drawings.
Magazine catch — flat plate with pivot hole + magazine-lip engagement surface. ~3 mm thick from 4140.
Internal pins — 2.5 mm and 3 mm ground music wire (ASTM A228) cut to length.
Springs: FCG springs are music wire (ASTM A228) wound on a mandrel; spec the wire dia and turn count per the Vol III drawings.

Step B4 — Closed-bolt FCG fit-up

Assemble the FCG internal mechanism in the housing:

Insert pivot pins for sear, trigger, disconnector, firing-pin striker, magazine catch.
Install springs.
Function-test by hand (no bolt yet; you’re checking the linkage):
- Trigger pull → sear releases firing-pin striker.
- Disconnector resets sear after each pull (semi-auto reset behavior).
- Magazine catch holds an empty Sterling-pattern magazine in the magazine well.
Verify the firing-pin striker travel matches the Vol-III-spec primer-strike depth.

Step B5 — Housing-to-receiver fit-up

With the receiver tube (from Package A) in hand:

Position the housing under the receiver tube, aligning the magazine-well slots.
Insert M5 bolts through the receiver-tube underside holes into the housing.
Verify the bolts engage cleanly without forcing.
With the bolts snug (but not torqued), function-check the FCG with the receiver in place — confirm the firing-pin striker travel still clears the receiver bottom and doesn’t bind.

The Vol 8 § 8.4 cross-reference covers the FCG mechanics in more detail; Vol 7 trusts that the parts engage and the linkage works at the functional test.

7.6 Work Package C — The bolt + (new) firing pin (step-by-step)

Estimated time: 8–12 hours. The bolt is the second-most-critical fabrication after the receiver tube; getting the diameter + length + bolt-face geometry right matters for headspace, feeding, and extraction.

Step C1 — Cut the bolt blank

From 4140 round stock at ~32 mm OD (or slightly oversized for finish-machining):

Cut a blank ~215 mm long (5 mm length allowance).
Verify the bar stock is straight to ≤0.1 mm runout over the cut length; if not, sort through stock or center-drill ends and turn between centers.

Step C2 — Rough turn the bolt OD

Set up in the lathe between centers or in a 3-jaw chuck:

Center-drill both ends.
Face both ends to length: 210 mm final.
Rough-turn the bolt OD to ~32.5 mm (0.5 mm finish allowance).
Verify OD with calipers along the length.

Step C3 — Finish-turn the bolt OD

Finish-turn to 32.0 mm ± 0.05 mm — this is the critical sliding-fit dimension against the receiver tube ID (~35 mm from Step A6, giving ~1.5 mm radial clearance — verify against your specific receiver-tube ID measurement). The bolt should slide into the receiver with hand pressure and no binding.

Step C4 — Machine the bolt-face features

With the bolt in the lathe, face the front end and cut the bolt-face features. The closed-bolt design adds a separate firing pin (vs the WWII fixed-firing-pin design); plan the bolt-face geometry accordingly:

Bolt face — flat surface perpendicular to bolt axis, recessed ~3 mm into the bolt OD to form a small chamber-mouth pocket for the cartridge head.
Firing-pin clearance hole — 2.5 mm dia hole through the bolt face’s center, extending ~15 mm into the bolt body to accept the separate firing pin.
Extractor slot — longitudinal slot in the bolt face perimeter, ~5 mm × 3 mm, on the right side (when the bolt is oriented with the ejection port to the right).

The separate firing pin is a small turned part: 4140 round stock at 2.4 mm dia, ~20 mm long, with a flange at the rear that retains it in the bolt body and a point at the front that protrudes ~1.4 mm beyond the bolt face. A small spring (music wire ASTM A228, ~5 mm OD coil) returns the firing pin to its retracted position after each strike.

Step C5 — Machine the rear-end features

With the bolt reversed in the lathe (using the previously-turned OD for chucking):

Sear engagement surface on the underside — a flat surface machined into the bolt OD ~15 mm from the rear, ~10 mm long × 3 mm deep. This is the surface the FCG sear contacts to hold the bolt in the cocked position. In the closed-bolt design, the sear holds the bolt in the forward position (chambered cartridge, hammer cocked, firing pin retracted) ready to strike on trigger pull.
Cocking handle pin hole — transverse hole through the bolt OD on the right side, ~50 mm from the rear, 6 mm dia, drilled with the bolt mounted in a milling fixture.
Recoil spring guide step at the rear — turn the rear ~25 mm of the bolt to a smaller diameter (~22 mm OD) to allow the recoil spring to slide over it.

Step C6 — Case-harden the bolt face + firing pin

The bolt face and the small firing pin need case-hardening for durability (Vol 4 § 4.4.3):

Pack carburizing — pack the bolt face in a steel can with carburizing compound (Casenit or equivalent), heat to ~900 °C for 60–90 min, quench in water.
OR salt-bath carburizing — gunsmith specialty; produces a deeper, more-consistent case.
OR low-temperature surface hardening — the alternative for the FCG sear contact (~Rc 55), using a torch + Kasenit applied locally.

Post-hardening, the bolt face should have a surface hardness of Rc 55–58 over a depth of 0.5–1.0 mm. The bolt body interior remains at the 4140 base hardness (~Rc 30).

Step C7 — Bolt verification

Slide the bolt into the receiver tube — should slide freely with no binding.
Verify the cocking-handle hole alignment with the cocking-handle slot in the receiver (Step A8).
Test-engage the bolt’s sear surface with the FCG sear (from Step B4) — should engage and release cleanly.

7.7 Work Package D — The barrel (chamber-in-house, rifling-farm-out)

Estimated time: 4–8 hours. The barrel is the simplest fabrication once the pre-rifled blank arrives.

Step D1 — Acquire the pre-rifled blank

Order a 9×19 barrel blank from a US barrel-blank vendor:

Length: ~250 mm (10″) — gives margin for muzzle-end fitting and chamber-end facing.
OD: ~13–15 mm (the Mk II barrel OD is dictated by the receiver-tube chamber boss; size to fit your specific receiver-tube ID at the muzzle-bushing location).
Bore: .355″ (9.0 mm), 4-groove right-hand rifling, 1:10 inch twist.
Material: 4150 chrome-moly (standard) or 416R stainless (alternative).
Vendors: E.R. Shaw, Green Mountain, Bear Creek Arsenal, McGowen, McCory. Cost ~$80–150 per blank.

Step D2 — Face the chamber end

In the lathe:

Chuck the blank with the chamber end protruding.
Face the chamber end to a perpendicular shoulder.
Verify the bore-to-OD concentricity with a dial indicator.

Step D3 — Cut the chamber

With a 9×19 SAAMI chamber reamer (Brownells, PTG, Manson — ~$200 for the reamer):

Set up the chamber reamer in a floating reamer holder (essential — a rigid setup will produce an off-center chamber).
Lubricate generously with cutting oil.
Run the reamer at low RPM (~50–100 RPM) with steady feed pressure.
Withdraw frequently to clear chips (every 3–5 mm of feed).
Verify chamber depth against the headspace gauge as you approach the final depth.

Step D4 — Verify headspace

Insert the GO gauge:

GO must close in the chamber and the bolt must close on the GO gauge without forcing → headspace is on the GO side correctly.

Insert the NO-GO gauge:

NO-GO must NOT close — the bolt should not be able to close on the NO-GO gauge → headspace is within tolerance on the NO-GO side.

If GO closes and NO-GO closes: chamber is too deep; the barrel is scrap or requires re-cutting the barrel face (with breech offset compensation). If GO does not close: chamber is too shallow; ream a tiny bit more (0.05 mm at a time, re-checking).

Step D5 — Cut the muzzle end

In the lathe:

Cut the muzzle end to total barrel length of 197 mm (Mk II spec — adjust if Vol 7 builder prefers a longer barrel to avoid the SBR posture).
Cut the muzzle bushing-fit shoulder (Step E2 reference dimensions).
Crown the muzzle with an 11° crowning cutter or a piloted muzzle-crown tool.

Step D6 — Barrel verification

Bore-mirror or borescope the chamber for surface finish (should be smooth, no machine swarf, no rough spots).
Run a chamber-clean swab to verify chamber cleanliness.
Verify the muzzle crown geometry with a magnifier.

7.8 Work Package E — Assembly, fitting, surface finishing

Estimated time: 8–20 hours for a first build (mostly fitting + finishing).

Step E1 — Mount the muzzle bushing

The muzzle bushing is a small lathe-turned part that threads into the front of the receiver tube and retains the barrel:

Cut from 1045 or 4140 bar at ~38 mm OD (matching the receiver-tube OD) and ~15 mm thick.
Bore the bushing for the barrel fit (per Step D5).
Thread the bushing OD for the receiver-tube interior thread (cut a matching thread on the receiver tube interior at the muzzle end with a lathe-mounted single-point internal threading tool, or fit a pinned bushing as the WWII alternative).

Step E2 — Install the barrel

Slip the barrel into the receiver from the muzzle end.
Seat the barrel against the receiver-tube chamber boss.
Thread (or pin) the muzzle bushing in place.
Verify the barrel is fully seated by extracting any play with hand pressure on the muzzle.

Step E3 — Install the rear cap

The rear cap is a threaded plug (left-hand thread per Vol 4 § 4.3.1) that closes the rear of the receiver tube and retains the recoil spring:

Cut from 1045 or 4140 bar.
Thread for the receiver-tube interior (left-hand thread).
Install with a small amount of thread-locker (Loctite Blue — not Red; you’ll want to remove it for cleaning).

Step E4 — Install the extractor

Pin the extractor (a small bent-spring-steel claw, fabricated in Package C or purchased as a Sterling-pattern aftermarket part) into the bolt’s extractor slot. Verify the claw engages a cartridge rim with light pressure but releases cleanly.

Step E5 — Install the cocking handle

Thread or pin the cocking-handle knob onto the bolt’s cocking-handle pin (Step C5). Slide the bolt into the receiver; verify the cocking-handle rides in the receiver-tube slot and can be drawn rearward to cock the action.

Step E6 — Install the FCG

Bolt the FCG housing (with internal mechanism installed) to the receiver tube underside. Function-test with a Sterling-pattern magazine inserted (empty):

Insert mag → magazine catch retains it.
Cock the action → bolt draws rearward, sear holds it.
Pull trigger → sear releases, bolt slams forward, firing-pin striker fires the firing pin.
Pull trigger again (no cycle yet — manually return the action) → disconnector resets, sear engages, trigger fires the firing pin again.

Step E7 — Install the stock

For the Mk II skeleton T-frame:

Fabricate from 9.5 mm 1018 round bar bent into the T-shape and welded at the joint.
Slide onto the receiver-tube rear lug.
Verify the cross-pin engages and holds the stock in place.

For the Mk V wooden stock (stretch build):

See § 7.12.1.

Step E8 — Surface finish

Per Vol 9 — parkerizing is the recommended finish for the Mk II Vol 7 build:

Strip all metal parts of oil and surface oxide.
Submerge in manganese phosphate solution at ~95 °C for 15–30 min until the parkerized surface is uniform.
Rinse + oil with a light-petroleum-distillate oil (any clean petroleum lubricant).
Verify the surface is uniform and consistent across all parts.

For the stock (Mk V wood path): Boiled linseed oil rubbed in over multiple cure cycles.

7.9 Test fire & break-in

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

Dry function check — close-bolt-style action, manual cycle 10×, verify FCG reset cycle.
Single-round chamber-and-fire — load one round in a fresh aftermarket Sterling-pattern magazine, chamber, verify cartridge is fully seated, point in safe direction, fire.
5-round semi-auto — verify the gun cycles, feeds, fires, extracts, ejects.
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 it appears, replace the magazine — see Vol 11 § 11.3).
Clean and inspect — disassemble after the heat-soak test, clean per Vol 11 § 11.7, inspect for any visible wear, fit, or damage.

7.10 Common pitfalls and how to avoid them

Receiver-tube seam off-center post-roll — caused by uneven roll pressure; correct by re-rolling with the wrap-spring-back compensated, or by hand-shaping with a soft hammer + mandrel.
Chamber too deep (NO-GO closes) — see Step D4; barrel is scrap or face + reseat.
Bolt too loose in receiver (>3 mm radial clearance) — bolt was turned undersized; either re-make the bolt or accept the loose fit if function-testing shows no issues.
Bolt too tight in receiver (binding) — bolt is oversized; relieve by careful sanding (use a polishing wheel on the lathe with the bolt mounted between centers).
FCG sear over-engages — sear pivot is in the wrong position; adjust per Vol 8.
Disconnector fails to reset — disconnector spring is too weak or the disconnector pivot is binding; re-fit.
Feed jam every round — feed-lip pathology in the magazine; switch to a fresh Sterling-pattern aftermarket magazine and re-test.
Cracked weld bead on the receiver tube — under-fused weld; grind out and re-weld + stress-relieve.

7.11 What could go wrong — failure modes

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

Out-of-battery fire — firing pin protrudes too far; reduce firing-pin tip protrusion; verify bolt-face geometry. Vol 11 § 11.5.
Light primer strikes — firing-pin spring too weak, or firing-pin tip too short; replace spring or re-make pin. Vol 11 § 11.5.
Stovepiped cases (case in ejection port) — extractor too weak or wrong angle; re-fit extractor. Vol 11 § 11.5.
Failure to feed (round nose strikes chamber) — magazine feed-lip geometry off; switch magazine or re-form. Vol 11 § 11.3.
Failure to extract (case stuck in chamber) — chamber too tight, extractor too weak, or both; verify SAAMI chamber dim, check extractor strength. Vol 11 § 11.5.
Failure to fire (sear/firing-pin striker disengagement) — FCG sear over-engaged or firing-pin striker travel-limited; check FCG geometry. Vol 11 § 11.5.

7.12 Stretch goals — Mk V conversion and select-fire NFA option

7.12.1 Mk V wood-furniture conversion

Once a Mk II build is working, the Mk V furniture upgrade is a stretch build that uses the same receiver tube + barrel + FCG-housing but replaces the skeleton-T stock with a wooden buttstock + foregrip + Lee-Enfield No 4 rear sight + bayonet lug.

Op summary:

Buttstock: CNC-router walnut blank from hardwood stock to the Mk V buttstock profile; install steel-cup buttplate.
Foregrip: CNC-router walnut blank from the same hardwood to the Mk V foregrip profile; clamp under the barrel.
Rear sight: source a Lee-Enfield No 4 rear sight as a standalone part from the Enfield-rifle aftermarket; install onto a sight-base machined into the receiver-tube top.
Bayonet lug: weld onto the muzzle bushing.

Estimated additional time: 8–12 hours.

7.12.2 NFA Class III pre-86-registered full-auto option (Path A combined with Path C)

If Jeff has a Class III SOT or works with a Class III dealer who has the pre-86 transferable Sten in inventory, the Path A registered transferable can be combined with Path C fabrication for everything except the registered receiver. This gives a select-fire Sten at the Path A cost minimum but with Jeff-fabricated barrel, bolt, and FCG.

Legal posture: the registered receiver is the gun for NFA purposes; the rest is replaceable parts. ATF will recognize the Jeff-fabricated bolt + barrel as service parts on the registered transferable receiver as long as the registration is properly recorded. See Vol 10 § 10.2 for the NFA Class III posture.

Estimated time: same as a Mk II Vol 7 build (Packages A through E) minus Package A (the receiver tube — replaced by the registered transferable receiver).

7.13 References (Vol 7)

Vol 4 of this series — Engineering & Reference Data; the dimensional source for everything fabricated in Vol 7.
Vol 9 of this series — Materials & Finishing; the surface-treatment authority for Step E8.
Vol 10 of this series — Legal & Regulatory Posture; the legal authority for the closed-bolt-only requirement and the manufacture-for-personal-use exemption.
Vol 11 of this series — Live-Fire Operation & Use; the test-fire procedure for Step 7.9 and the failure-mode diagnosis for § 7.11.
Professor Parabellum. Practical Scrap Metal Small Arms, Volume III: The DIY STEN Gun. (Wikimedia Commons, CC BY 1.0 PDF.) The canonical modern reverse-engineered op-sequence treatment + closed-bolt FCG layout. The Professor Parabellum drawings are the dimensional source for the closed-bolt FCG features in this volume’s Step B2; cross-check against the Vol III drawings for the specific firing-pin striker pocket dimensions before machining.
Bear Arms. Closed-bolt FCG layout for a US-market semi-auto STEN. (Modern community reference.)
UK Ministry of Supply. SA-prefix drawings for Carbine, Machine, 9 mm STEN Mk II. (National Archives Kew + Royal Armouries Leeds.) The dimensional source for receiver-tube + bolt + barrel + magazine geometries.
Brownells. Chamber Reamer + Headspace Gauge Catalog. (Vendor reference for Step D3 + D4.)
Sheet Metal Forming Handbook. (General machinist reference for receiver-tube rolling.)
Machinery’s Handbook. (General machinist reference for the lathe + mill operations throughout.)
South Bend Lathe Works. How to Run a Lathe. (Free PDF online; covers the bolt-turning and chamber-reaming operations.)