BTW Interestingly, the following quote from this article about the Birmingham Gun Proof House..Clicky... mentions just this problem.
The next room was a sort of 'Black Museum' of what can go wrong if firearms are misused, a P14 rifle split from end to end from carelessly leaving a cleaning patch in the bore, a 25/06 rifle that had had a .308 round fired in it. After looking at the exhibits here anyone with a shotgun that has interchangeable chokes will ensure that they are fully screwed home. Some of the more interesting examples were the result of some experiments carried out in the proof house on the effects of first loading a 20 bore shell ahead of a 12 bore round and firing it. The only gun that seemed capable of handling this appeared to be the Russian Baikel. High speed photography showed that the barrel bulged to release the pressure but was sufficiently elastic to return to its original form and suffering no lasting effects. Not to be recommended practice for Baikel owners all the same!
