Cartridges up to 19.1mm, shotgun shells
Quantity limits: Reasonable amount for "personal use" during a trip. However, international (ICAO/IATA) regulations and some airlines in the U.S. limit this to 5 kg (11 pounds) gross weight per passenger. Check with your airline.
Small arms ammunition includes cartridges up to 19.1 mm (.75 caliber) and shotgun shells. It does not include black powder, smokeless powder, primers, percussion caps, or homemade powder and ball loads for muzzle loading.
Ammunition must be securely packed in boxes or other devices specifically designed to carry small amounts of ammunition. Ammunition clips and magazines must also be securely boxed so no ammunition is loose or exposed.
Loaded firearms are forbidden.
These restrictions do not apply to armed law enforcement officers traveling under the conditions set forth in 49 CFR 1544.219.
See the regulation: 49 CFR 175.10(a)(8)
See TSA's security screening requirements for ammunition and unloaded firearms in checked baggage.