General Discussion

Current law says a gun cannot be shipped through the mail, but a "gun part" can be.

The Bureau of Alcohol, Tobacco and Firearms - which sounds like the name of a Wyoming convenience store - says a "gun" is the part of the gun with the serial number on it.

If you have an AR-15 or other Modern Sporting Rifle/"black gun"/assault rifle, the serialized component is the lower receiver casting.

An automatic pistol's "gun" is the frame, which is the part that features the handgrip.

A popular hobby in the shooting sports is to buy an AR-15 lower receiver casting from a gun store, then assemble the weapon from parts you chose individually.

However! "Stuff like silencers" are National Firearms Act items, "Class III weapons" or "Title II weapons." They fall into the same legal framework as machine guns, except that there's a law that set an end date for manufacture of machine guns transferrable to civilians and new suppressors are legally made every day. Parts to turn rifles into full-auto weapons are themselves considered machine guns, and the only way you can legally possess or transfer one is if it was installed into a rifle before the May 1986 cut-off date.