A few more notes on this go/no-go gage. The inside dia of a 12ga hull will be some larger than the standard bore of the gun (.729") often around .750". The head of a 20ga will go to about .700", definitely not large enough to provide a resistance to the end of the 12 ga from enterong the cone. In all the case I tried this on if the 20ga rim actually entered the 12ga mouth it then became too tight to reach the end of the chamber. The only way Ican see for it too work at all would be if the 12ga hull mouth had just the right amount of skiving for the rim of the 20 to just seat inside the end. None of the combinations I tried proved to be of any value whatever. In placing the cut down 20 into the mouth of a fired 12ga crimp memory is what keeps the 20 from just sliding freely.
I don't recall now what hulls I had on hand when i tried this several years ago upon first hearing it, but on all the ones I tried when the rim was pushed inside the 12 the od was swelled to over the .800" dimension & on some would go no more than about half the chamber depth. I quickly lost all interest in the method. It is noted also that Parker is one of the American companies which has often been mentioned here as one who intentionally chambered their guns 1/8" short of nominal shell length, thus that 2 5/8" chamber is probably right for a gun intended for use with 2 3/4" shells.
