My (1862) Purdey 20-bore is stamped 22 on the flats and noted as a 24-bore in the leger and it likes .625" pure lead ball or alloy ball.

My (1864) Westley Richards 16-bore is stamped 17 *and* 18 on the tubes just forward of the flats, and is noted as a 16-bore in the leger.

The Purdey (should check my notes) has a .623 or so bore, the WR somewhere in the .650's for bore (octagonal Whitworth patent profile) and grooves in the .680's (again should check my notes) does well with .690 ball

Cheers
Tinker