How Reliable are Homemade Firearms from 80% Lowers?

“...I always wondered why the ATF chose the lower to make 80% and not the upper...” [Abathar, post 5]

Because by law, administrative regulation, and precedent, one single part must be “the gun” in a legal sense, bearing the serial number, model designation, and other markings.

In a great many cases, the obvious choice was the receiver or frame - typically the largest, most difficult to make, and the most expensive.

Not every firearm conforms to the pattern. The most commonly cited example is the Whitney Wolverine: no frame, lots of small parts.

FN’s FAL is the opposite of the AR-15: upper receiver bears the serial number, lower doesn’t.

Few gunmakers bothered with serial numbers until the War Dept began to obsess over inventory control after the American Civil War. Prior to the Industrial Revolution, guns were one-off products of skilled craftsmen; a gun made in a factory as one of thousands by series production was considered downscale and in poor taste, owned only by users too poor to obtain better, or too lacking in taste to known the difference. Many early revolvers had the serial numbers stamped on the grip straps where the grip panels would hide them.

Serial numbers were not required on all US-made guns until the 1968 Gun Control Act took effect. Large numbers of rimfire rifles and bolt action shotguns are still on the market without serial numbers, having been made in the 1950s and 1960s.

I saw someone mentioned using a router jig from 5D Tactical. From personal experience, a drill press jig is more stable, precise, and easier to use for cutting the main fire control cavity compared to a router - this is especially true for newer builders. Router jigs see wear-and-tear quicker (due to tool on tool contact of the base plate, adapter plate, and router itself) and starting each cutting pass creates a high risk of bit chatter and essentially "gouging" parts of the receiver you don't want to touch. A drill press 80% lower jig is a better option, overall. A drill press can't be side-loaded like a router or mill, but using plunge cuts produces a smoother finish with much lower risk of making a mistake.

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