Ghost Gun CNC: How DIY Firearm Machining Actually Works

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  Key takeaways
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  What an 80% receiver is and why CNC is used
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  How ghost gun CNC steps run on a benchtop mill
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  Where DIY receiver machining stops working
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  What a compliant CNC shop checks before quoting
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  A practical setup sequence for a lower-style pocket
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  Which method fits which receiver blank

A ghost gun is a firearm assembled by an individual with no serial number and no commercial manufacturer of record. In practice, most builds start from a partial receiver, often called an 80% lower. It arrives with the fire-control cavity unfinished and no serial. The buyer removes the remaining material, then adds the trigger group, barrel and stock.

The unfinished portion is a pocket. On an AR-style aluminum lower, that means removing roughly 20 cm³ of 7075 or 6061 from the trigger well, then drilling the selector and hammer pin holes. On a polymer lower, the same pocket comes out with a router or drill press. Steel and titanium versions exist but are rare because they wear tooling fast and offer no functional gain.

That is why ghost gun CNC work looks familiar to anyone who runs a job shop. It is 2.5D pocketing with a few cross holes. A three-axis mill with a 6–8 mm carbide end mill handles the cavity. The pin holes need a drill or a boring cycle, not a five-axis move. There is no undercut, no deep rib, no thin wall that needs support.

The reason a machine is involved at all is repeatability. A hand drill wanders. A pin hole that is 0.15 mm off center changes the trigger geometry and can cause light strikes. A controlled spindle, a fixture and a pre-set depth make the same cut every time. That is a machining argument, not a firearms argument, and it applies to any pocket-and-hole part.

The sequence on a small mill is short. Fixture the lower so the magazine well is square to the spindle. Rough the fire-control pocket in two or three depth passes. Semi-finish, then finish with a smaller end mill and a light radial stepover. Drill the pin holes. Deburr. That is the whole job.

Roughing is where most of the time goes. A 6 mm three-flute carbide end mill in 6061 runs comfortably at 8,000–12,000 rpm with a 0.05 mm/tooth feed. Depth of cut around 1–2 mm per pass keeps the tool from grabbing in a deep pocket. For 7075, drop the surface speed about 25% and expect more chatter if the fixture is light.

Pin holes are the accuracy-critical feature. Drill them 0.05 mm undersize, then ream to size. Center tolerance of ±0.05 mm is realistic on a benchtop machine; ±0.02 mm needs a rigid setup and a probe. Anything looser and the trigger group may not time correctly. Hole position matters more than hole diameter here.

Chip evacuation decides surface quality. Aluminum packs into a blind pocket fast. Use air blast, peck the drill, and stop to clear. A finished pocket floor at Ra 1.6–3.2 μm is normal and fine; the trigger group does not care about a mirror finish. Do not chase Ra 0.2 μm on a non-functional surface.

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  6061-T6 is the defaultEasy to cut, stable, cheap. 7075 gives more strength but loads the tool faster.
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  Fixture before you cutA soft jaw or a dedicated plate. A vise alone lets the part walk on deep passes.
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  Ream the pin holesDrill undersize, ream to size. This is the cheapest accuracy you can buy.

The 80% concept works because one pocket is left out. It stops working the moment the part needs a serialized component or a controlled operation. A completed receiver is a firearm. Whether you may finish it, register it, or transfer it depends on where you are, and in the US the rules have changed more than once in the last few years.

There is also a machining boundary. Polymer lowers crack at the buffer tower under repeated recoil when the wall is thin. Cast aluminum with porosity fails at the pin holes. Neither is a CNC problem you can fix with better tooling; the raw stock is the limit. Billet 6061 or 7075 does not have that issue, which is why most serious lowers are billet.

For a contract shop, the harder boundary is legal, not technical. In the US, ITAR restricts exporting firearms technical data, and a CNC program for a receiver can count as technical data. Manufacturing a firearm part for a customer without the right federal license is not a small compliance gap. A shop has to know what it is making and for whom.

That is why we screen work by end use. GreatLight machines aerospace, automotive, medical, robotics, energy and industrial parts. We quote firearm components only where the customer holds the required license and the paperwork is clean. A five-axis center that cuts a titanium bracket all day can also cut a receiver, but that does not make it a lawful job.

A shop that takes this kind of work seriously asks three things up front: what the part is, who the end user is, and what license covers it. If the answer is vague, the job does not go on a machine. That is not caution for its own sake; a receiver program crossing a border can create an export-control problem that costs more than the job.

After that, it is normal engineering. Material certificate. A DFM pass to flag thin walls and deep pockets. A fixture plan. For a receiver-shaped part in 7075, we would check wall thickness against the recoil load, confirm the pin hole positions in the drawing, and decide whether one setup on a five-axis center beats two setups on a three-axis mill.

For non-firearm work, the same pocket-and-hole geometry appears constantly: aerospace brackets, robot end-effectors, medical instrument housings, EV battery trays. The tooling, feeds, tolerances and inspection logic are identical. That is the useful takeaway. Ghost gun CNC is not a separate discipline; it is a small, legally constrained slice of ordinary pocket milling.