United States Code, Title 18, Part 1, Chapter 44, firearm definitions includes: any weapon (including a starter gun) which will, or is designed to, or may readily be converted to expel a projectile by the action of an explosive; the frame or receiver of any such weapon; any firearm muffler or firearm silencer; and any destructive device. As defined by 49 CFR 1540.5 a loaded firearm has a live round of ammunition, or any component thereof, in the chamber or cylinder or in a magazine inserted in the firearm.
When traveling, comply with the laws concerning possession of firearms as they vary by local, state and international governments.
Declare each firearm each time you present it for transport as checked baggage. Ask your airline about limitations or fees that may
Firearms must be unloaded and locked in a hard-sided container and transported as checked baggage only. Only the passenger should retain
the key or combination to the lock.
Firearm parts, including magazines, clips, bolts and firing pins, are prohibited in carry-on baggage, but may be transported in
Replica firearms, including firearm replicas that are toys, may be transported in checked baggage only.
Rifle scopes are permitted in carry-on and checked baggage.
More than that they do not seem willing to disclose, though they do offer:
Contact the TSA Contact Center with questions you have regarding TSA firearm regulations and for clarification on what you may or may not transport in your carry-on or checked baggage.
Allowing rifle scopes in carry-on luggage implies to me these are not seen as "firearms", despite being under that heading. On the other hand the fourth bullet point implies to me that magazines, clips, bolts and firing pins are seen as "firearms", perhaps either under "may readily be converted to expel a projectile by the action of an explosive" or "the frame or receiver of any such weapon; any firearm muffler or firearm silencer".