Well I'd think a 20 ga shell in a 12 barrel might be classified as an obstruction.

Bob, yes I really did need to blow up a shotgun barrel with the 12/20 trick for the Hunter Ed students. I was given a Browning 300 Win Mag rifle barrel split into a "y" right back to the front of the chamber, snow in the muzzle.
I might try to blow up the JABC twist barrel with a 3" slug and a 20 ga shell.
Mike