What Miller said
A study by the Royal Military College of Science, sponsored by the Birmingham Proof House and the British Association for Shooting and Conservation, showed that an obstruction by 2 fibre wads (total weight of 4 grams) was sufficient to bulge or burst a 12 gauge barrel shooting a 28 gram (slightly less than 1 ounce) load. Peak pressure occurred 22mm (.866”) past the leading edge of the obstruction.
And more from Burrard
If the burst was not caused by an obstruction it must have been the result of some excessive pressure or of some abnormal weakness in the barrel.
A (excessive load) pressure burst can only occur in the immediate neighbourhood of the chamber; and so if the burst occurred ahead of the chamber cone an excessive pressure can be ruled out.
Another chamber burst
BUT an asymmetric ring bulge related to the addition thickness of the brazed barrel flats and support of the right barrel. The burst was likely a wad in the forcing cone
Federal 10g with 2 piece fiber base wad
