Tested my 24ga.

Like Fly hamps the chamber will not let a 20 hull in at all. Also tested 3 different sets of snap caps and none would make it past the rim recess into the chamber.

The rim recess measured .727, the top of the interior of the chamber measured .694.

If you can get a 20 gauge snap almost all the way into the chamber i suspect it may be a 20 with a very tightly cut chamber, similar to the difference between two of my 16s one of which will only let a fired all brass shell into the chamber to the last 1/8 of inch or so

Michael Dittamo
Topeka, KS