Thank GOD he went through extra security measures at the airport which are specifically designed to catch things like this... Wait what?
From the link:
A man accidentally brought his loaded .40 caliber gun onto an airplane. He didn't know he had it. Neither did the TSA, who completely missed it during its scan and allowed that plane to fly.
Source -> http://abclocal.go.com/ktrk/story?section=news/local&id=7848683