I found this on the united page. I am sure it is similar for all carriers.


Federal regulations require that firearms in checked baggage or in their own shipping cases be unloaded and declared. Firearms are not allowed in carry-on bags. Firearms in checked baggage must be packed in a locked hard-sided bag or gun case. TSA locks are not approved for securing firearms. A maximum of 11 lbs. of small arms ammunition is allowed in checked luggage in the original manufacturer's box or securely packed in a fiber, wood or metal box to prevent movement of cartridges.