The measurements in Drew's picture don't look like "blow-up" thicknesses. Back to the obstruction theory.
People shoot old ammo or load ammo to low pressures, therefore, obstruction should not be ruled out.
I have simple rule if 12ga gun can't handle 1oz game load at about 1250fps or 7/8oz for 20ga I dont want to own it or to use it.
Classic guns are cheap now. Last one I looked at was keeper gun from Birmingham by English & Son (very prestigious shop from Bath Street). It had 28" imp cyl-mod barrels 2&3/4" chambers with 1&1/4oz proof, $500 plus tax.
