I assume that we are talking breech loaders? If so ,13 = .710 . 13/1 = .719 . A gun proofed under British law goes out of proof at the next bore size up .
We found this to our cost when guns like the early Miroku's came in bored to 18.5 mm that's .728 in real money . They were plugged and as they would not take a .729 plug they were stamped at .719, thus only having .001 in proof. After this was discovered the practice was to use an intermediate plug of .725. If the barrel accepted this then it was marked .729 .If not it was marked at .719 .This was only applied to imported guns.
