2-Piper,I would agree that the accidental insertion of a 28 G shell in a 20G gun could result in the obstructional burst pictured.Burrard in Volume 3 of his book,The Modern Shotgun",pages 380-381 describes this possibility and states;"From the very nature of the obstruction the burst must always occur a little distance in front of the chamber cone."
Exactly the failure seen in the gun pictured.Obstructional bursts create a wave pressure that is much higher than that developed in the normal discharge of a shell in a barrel free from obstructions.We can see the resultant pressure increase totally destroyed the gun.What a lucky escape!
