On the 12-20 bursts there are two factors at work. First the 20 rim will hang in the forcing cone so there is a bit of clearance for an obstruction situation, however on the other hand it is so near the 12 that actual chamber pressure is raised as well. These bursts will be in the chamber/forcing cone area. The rim on a 20 gauge shell runs around 0.760". This will go beyond the 0.798" diameter of a normal chamber end of the 12 but will not enter the bore of even an 11 gauge @ 0.751", but will stop at some point in the cone dependent on exact angle of the cone. it will not enter any bore except something on the order of a chamberless gun. A barrel which bursts out beyond the hand doesn't sound ike a typical 12/20 burst but purely an obstructional burst, "Me Thinks" that one needed a lot more investigation.
