BTW: 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.
Note this statement in Drew's post. This would explain why the two bulges are not exactly aligned. Burrard explains it by the fact when the charge hits the obstruction & is checked by it, it is in fact still moving very rapidly. The charge has actually moved forward before the pressure stacks up sufficiently to cause the bulge. This would seem to be the exact situation here as the ring in the top barrel is slightly forward of the one in the lower barrel.
Drew & I Both seem to concur on the solution what more do you want
