It is admirable that you would alert us, the NRA, and USCPSC about this potential problem, but I think you may be misunderstanding what Daryl was asking.
The body of the snap cap should be just under chamber diameter, and the chamber diameter is larger than nominal bore diameter. So even if the rim of the snap cap is totally gone, the body of the snap cap shouldn't go past the forcing cone and all the way into the bore... unless the bore is quite a bit larger than nominal bore size for a 20 gauge (.615") or the body of the snap cap is considerably under the normal chamber diameter.
Actually, a snap cap that had a
minimum body diameter of .675", or .010" under the normal diameter of the front of a 20 gauge chamber, shouldn't be able to fall all the way into a nominal
16 gauge bore with a diameter of .662". I would expect it to go well into the forcing cones, but it shouldn't be possible to fully chamber a loaded shell behind it. Your Pachmayr snap caps must be very much under chamber size if it is sliding into a 20 gauge bore.
