Burrard reported on obstruction tests which were done with a hollow obstruction which resulted in as severe a bulge as an equal weight solid one. This definitely showed that it was not the trapped air ahead of the charge that caused the bulge, but the Gas Hammer behind it as Don said.
He also reported on some cases where the bulge was closer to the breech but not severe enough to cause a burst. In some of these tests two or even three ring bulges were produced. Each moved further down the barrel & were less severe than the first one. This was explained as the result of those Shock Waves Don mentioned as they bounced back to the breech, then forward again to catch up with the still moving charge & produce another bulge. These tests were performed in a barrel tapped for a crusher pressure gauge. In no case was the initial max chamber pressure increased from that of the normal shell being fired. Firing a 12 gauge shell with a 20 gauge one lodged in the forcing cone would raise the chamber, but an obstruction further down the barrel does not, just creates that localized pressure hammer.
