Four definitions from § 478.11 and § 478.12 carry the entire regulatory system that later chapters build on: what counts as a firearm, what counts as ammunition, what specific part of a weapon is the frame or receiver, and when an unmarked weapon someone built at home becomes a privately made firearm. Every recordkeeping entry, every transfer, and every inspection finding traces back to whether the object in question fits one of these definitions — get the definition wrong and everything downstream is wrong with it.

The firearm definition is deliberately broad: a weapon that expels a projectile by an explosive, the frame or receiver of that weapon, a firearm muffler or silencer, or a destructive device — but not an antique firearm, and not a weapon whose frame or receiver has been destroyed in the manner the frame-or-receiver definition itself describes, which is where the regulation puts that test rather than in a definition of its own. Critically, the definition also reaches a weapon parts kit that is designed to or may readily be completed, assembled, restored, or otherwise converted to expel a projectile. That single clause is why a box of parts on a shelf can already be a firearm before anyone has assembled it.

The frame or receiver is the specific part the regulation treats as the firearm itself when a weapon is disassembled: on a handgun, it is the part providing housing or structure for the component — the sear or its equivalent — designed to hold back the hammer, striker, bolt, or similar primary energized component before the firing sequence begins. A sear is simply the catch that keeps that component under tension until the trigger releases it. On a rifle, shotgun, or other long gun, it is the part housing the bolt or breechblock. Ammunition, by contrast, is defined narrowly around the components — cases, primers, bullets, or propellant powder for use in a firearm other than an antique firearm — with specific carve-outs for loose shotgun shot and empty, non-metallic shotgun hulls without a primer. A privately made firearm ties the two together: a firearm, including a frame or receiver, produced by someone other than a licensed manufacturer and never given a serial number by one at the time it was produced.