I once received a used BP revolver through the mail inside of a medium flat rate box with a few inches of the barrel sticking out of the bottom corner of the box. Thankfully it wasn't damaged at all and when I saw the letter carrier he told me how concerned they were down at the post office about the contents. But he told the other postal employees not to worry because he figured that it wasn't loaded.
I guess that the lesson is to try to wrap the gun with something to cushion the sharp edges so that it won't bust through the box during handling. Or at least reinforce the inside of the box with some tape where it's rubbing.