http://www.united.com/page/article/0,6722,1035,00.html

Firearms
Federal regulations require that firearms in checked baggage or in their own shipping case 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. 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.