Hi Randy, the bore is .719 (or was at the time of proof). The 12C in the diamond is the gauge size for the gun. 12 the bore and C for chamber, not choke. There is also a LC which means long chamber. A diamond mark with a 12C would cover guns with a bore size of 12 or 13.

I messed up, the 13 over 1 won't be in a diamond, sorry, I got my wires crossed.

All the best!

Greg


Gregory J. Westberg
MSG, USA
Ret