L Brown'
Whilst you are correct in stating that 12 /C means that the chambers are less than 3 inches this mark must be read in conjunction with max shot load markings, in this example 1 1/8 max; ties into the scale of proof load used by the Proof House applicable to 2 1/2 or 2 5/8 chambered guns.[See the applicable British rules of proof]
A 2 3/4 or 2 7/8 inch chambered gun with the 12/C mark will be stamped 1 I/4 ozs; max for a corresponding maximum breech pressure of 7280 P.S.I.
Safety being the priority, it is vital that proof marks be properly understood if accidents are to be avoided.
Always measure the actual chamber length and bore sizes and compare with the the proof marks and applicable specified proof dimensions. It is also a wise precaution to measure minimum wall thickness of the barrels. .020 inches is considered to be a minimum.This feature is not specified in the rules of proof but indicates the durability and remaining life in the barrels.
Unfortunately many 2 1/2 chambered guns encountered in N.A. have had their chambers extended to 2 3/4 inches rendering the gun," out of proof" and thus a potential safety hazard.
This issues has had much attention in the past on this forum.It is clear that some 2 1/2 chambered guns were at the customers request tested to 2 3/4 pressures and marked 1 1/4 ozs; max.
