The questions by the OP were the wrong questions. All that does is make the other person with the gun; assuming they are generally a law abiding person; get defensive and want you to mind your own damn business.
Now, if you really "Cared" about this fellow gun owner, and weren't trying to act holier-than-thou; you simply should have said; "In case you didn't know, it's against the law to carry a gun in the post office". And left it at that. Forget about his ability to conceal a gun. That isn't as important as carrying in a post office. If you truly CARED, you would have mentioned that. If you were asking for personal gratification, you'd try to embarrass him. Which is what it sounds like.