Firearms, ammunition and powder in air travel
Firearms and parts at a US checkpoint
For a departure from the US, TSA prohibits firearms and firearm parts in carry-on baggage. An unloaded firearm can travel in checked baggage only in a locked, hard-sided case and must be declared to the airline at check-in. A starter pistol follows the same checked-bag preparation. Frames, receivers, magazines and other firearm parts have a separate TSA listing; check the airline's conditions before packing them. See sport weapons and replicas for lookalike items.
Boxed ammunition is not loose powder
Qualifying small-arms ammunition may travel in checked baggage if it is securely boxed and accepted by the airline. It cannot go through the passenger checkpoint. FAA's cartridge exception does not include black powder, smokeless powder, primers or percussion caps. Do not pack loose gun powder in either bag. Ask the airline about its quantity limit; the 5 kg limit applies under international rules and some US airline policies, rather than every US itinerary.
Check the airline and every jurisdiction
Confirm the airline's acceptance rules and the laws governing possession at departure, transit and destination before travel. Checked-bag permission depends on the exact item and its packaging. It does not by itself establish that you may possess the item throughout the route.
Sources: TSA: firearm transport, TSA: item list, and FAA PackSafe: ammunition.