I don't understand Craig.
The OPs burst is at about 9" and .040" is generally recognized as adequate wall thickness for fluid steel at that point.
The Smith Ideal chamber burst occurred where MWT at the end of the 12g chamber is usually at least .110". I did not however have the opportunity to measure that specific gun.
Another overpressure chamber blow out courtesy of David Trevallion
Likely 20g in front of 12g blow out
As said, bursts can occur with much less than total bore obstruction. 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.
Threaded Mode