For AR-15 platform rifles, the lower receiver (the part that houses the trigger, grip, selector lever and recoil/stock assembly is considered the gun. The upper receiver (receiver, barrel, bolt carrier group) is just a part.
Under federal law, you can legally build your own firearm as long as you don’t sell it. Ever. You don’t need paperwork or approvals. So, many AR enthusiests prefer to build their own lower and buy or build the rest. You can do this by purchasing an 80% completed lower and finishing it, or buy starting from scratch. There’s even a mold kit where you can cast your own AR lower out of plastic. So, naturally, it’s moved on to 3D printing.
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