If a pressure wave theory is valid, Doc Drew cited above that it can be reflected. Maybe, rivelling and other bulges can occur by random additive wave interference, and not necessarily an obstruction. Classic game guns may be intentionally struck or may acquire unintentional hoop strength reductions that just aren't seen as commonly in thicker and tougher modern barrels. Could that make them a bit more susceptible to a random wave amplitude that might approach double of what was intended?