Barrel Cutting, Crowning & Polishing

Inner barrel work is the difference between a build that groups at 4 inches at 50m and one that walks shots a foot wide for no obvious reason. The procedure is short but unforgiving — every cut and polish step has to leave the bore concentric and burr-free.

• Inner barrel (Arcturus 6.02 is our reference)
• Pipe cutter or fine-tooth hacksaw (pipe cutter preferred for square cuts)
• Fine file
• 400 / 800 / 1500 grit wet/dry sandpaper
• Cotton cleaning rod with patch holder
• Brasso (for brass; skip for stainless)
• Isopropyl alcohol
• Rotary tool with a small conical stone (crowning)

1. Confirm the target length against the muzzle device or suppressor that will live downrange of it. Measure twice.
2. Wrap the cut line with a single layer of masking tape — this gives the pipe cutter a clean track and protects the bore around the cut.
3. Score with the pipe cutter in light passes, advancing the wheel a quarter-turn between rotations. Heavy passes deflect the wall and oval the bore.
4. Once cut, remove the tape and inspect the cut face under magnification. Any roll-over of the bore lip is a failure — restart with a new cut.

The muzzle crown is the last surface the BB touches. An uneven crown destabilizes the BB the instant it leaves the bore.

1. Light the rotary tool conical stone at the lowest speed.
2. Apply the cone tip into the muzzle bore at perfect axis to the barrel. Two to three seconds is enough to cut a clean 11° internal chamfer.
3. Rotate the barrel one full turn against the spinning stone so the cut is even all the way around. Stop the moment a bright, continuous ring appears.
4. Wipe the crown with isopropyl and inspect. The chamfer should be uniform width all the way around. Any flat spot means more pressure was applied on that side — redo with lighter, more even pressure.

1. Wrap a patch around the cleaning rod, soak with isopropyl, and run the bore three times to remove cutting debris.
2. For brass barrels: wrap a fresh patch in Brasso, run the bore ten passes, then dry-patch until the patches come out white. Skip Brasso entirely on stainless — it leaves residue that builds up.
3. Final pass with a dry, clean patch. The bore should be mirror-bright end to end.
4. Optional: a single pass with 1500 grit on a tight patch will knock down any micro-burrs left by the cutter. Do not use anything coarser than 1500 inside the bore.

Reinstall the polished barrel only after the hop chamber is built per chapter 12.