A while back my dad sent his .270 Browning rifle to Browning to get the barrel replaced. I remember he called Browning as well as the post office concerning how to send it. He just sent it in reinforced cardboard tubing through the post office. Sending a pistol might be different concerning issues you may face. Just call a gun manufacturer like Browning, Winchester, etc. and the post office on how you should go about mailing it.