My nominal 12 gauge Greener hammer gun proofed under the rules in effect in 1879 had been honed out to .783 bores and accepted 2 7/8 ten gauge shells. It still had .110 wall thickness at the end of the forcing cone and minimum wall thickness at around .022 about 6 inches back from the muzzle. A 12 gauge shell was loose in the chamber but would not fall in.