Before the advent of the fractional bore measurements (13/1 etc), which was in 1887 (same time "not for ball" disappeared), you had a spread of about .020 for each gauge. If marked "not for ball" and not rifled, a 13--under the rules of proof prior to 1887--would have been in proof from .710 up to .728, out of proof at .729 (where the 12 starts).