Originally Posted by Ithaca5E
Those old guns were chambered to work with thick-mouthed paper shells. Mike a once-fired paper shell and then mike an AA or STS and you will see that the case mouth on the newer shell will not run interference with a 2.5" chamber.

I’m not sure I’m getting you. Are you talking about the thickness of the paper unfolded, being thinner now, so even a little overlap extension past the chamber doesn’t create too much pressure?