The video in post 50 is very interesting but it is hardly conclusive of all 20/12 situations. I witnessed a O/U blow up about twenty years ago. A buddy and I were shooting five stand and he found a unfired twenty gauge shell and he put it his shooting vest. About an hour later he blew his O/U up and the 20 gauge shell was no longer in his vest. We both believe that missing 20 gauge shot shell was the cause. Either that or space aliens beamed the shell out of his vest.