Cadet is correct, it's bore diameter. Also, keep in mind this does not indicate an "Exact" bore diameter. It meant that at time of proof the bore would accept a 40 gauge plug of .488" diameter but would not accept a 39 gauge plug of .492" diameter. The actual diameter of bore could have been anywhere in between.
The difference between your bore marked 40 & cadets marked 39 may be no more than a couple of thousandths. Even if they were both to exact size it would only be .004" difference.
Hello 2-piper,
Hello cadet,
Thanks for the reply.
So its possible that a .510 dia 50 cal bullet would still be a correct fit to function in a barrel stamped "40"? That could account for .011" deep engraving of a bullet. I suppose that would be OK for paper-patch lead bullets, but seems like it would be a little much if using Woodleigh jacketed 50 cal bpe bullets.
