I had a buddy do that and he was loading pistol and then went to 223. It was a brand new winchester featherweight gun. When he shot the first round it blew the top of the receiver off. He took it back to the store and they (winchester)gave him a new one. THen he traded the new one for a ruger and blew that up! He then realized what had happened. Talk about closing the barn door after the cows got out!