Here they a

12.1 Firearms

12.1.1 Definitions

The terms used in this standard are defined as follows:

[Delete current items 12.1.1a and 12.1.1g in their entirety and replace with the
following.]

a. Firearm means any device, including a starter gun, which will, or is designed to,
or may readily be convert*ed to, expel a projectile by the action of an explosive;
the frame or receiver of any such weapon; any firearm muffler or firearm silencer; or
any destructive device; but the term shall not include an antique firearm.

b. Firearm frame or receiver is the part of a firearm that provides housing for the
hammer, bolt or breech*block, and firing mechanism and that is usually threaded at
its forward portion to receive the barrel.

c. Handgun (including pistols and revolvers) means any firearm that has a short stock
and is designed to be held and fired by the use of a single hand and subject to
12.1.1a, or a combination of parts from which a handgun can be assembled.

A handgun is a firearm, and a 'firearm' includes the frame or receiver thereof. So
shipping 'parts' would be illegal if one of them was the frame or receiver.

Good idea though. Maybe 'parts' would work under UPS or Fedex rules, unless they use
the USPS definitions.
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Guns don't cause problems. The behavior
of certain gun owners causes problems.