I have tried to blow up guns for my hunter safety students to see. A Noble pump 12 ga and a cheap sxs Belgian Damascus hammer 12 ga
Each gun had a live 20 ga shell dropped in and a 12 ga target load and fired, repeated 3 times with no damage to the guns. Perhaps compressed air blew the 20 ga shell out and it did not fire?
I cannot really explain why the barrels did not let go.
Jamming some mud into the muzzles of the sxs produced a satisfying result, however, opened up a big flap of barrel/
Guns were tied to a round bale of hay with a long length of baler twine and me hiding behind another big bale.
Mike