....We were trying to figure out what the pressure might be....
....The intent is clearly to test not only the chamber but the forcing cone to a reasonable distance.
While we don't seem to know what pressure test point is being targeted at 162mm's from the breech, there does seem to be some agreement on what a shotgun shell pressure curve looks like. It might be that 'testing' downstream of the forcing cone is intended to exceed stated proof pressure at 17 and 25mm's.
Say, for discussion, that the second proof load firing is the 162mm test. I wonder what percentage of failures are racked up on that second shot. Rivelling, bulges and a cracked frame? Maybe, there's a possible reason why, 'tightening proof tests in recent years has made it a rather risky business'. Note, no anecdotal or otherwise reports of specific failures at 162mm's. I think other options could be reasonably guessed at.
