According to Greener in his book,"The Gun", 12/C was used to denote chamber lengths upto 3 inches. Above 3 inches the 12/lc mark was used. If a gun is nitro proof the shot load given is a good indication of chamber length;.For example
1 I/8 OZS;=2 1/2 chamber.1 1/4ozs;is almost always = 2 3/4 chambers.


Roy Hebbes