Owen;
In your original post on this gun, you had just stated that the bore was proofed at 11 with no mention of the chamber. I had been thinking all this time it was an oversize 12. It makes a lot more sense as a 10. That would certainly account for the heavier walls etc. Most likely it has been enlarged but very little from where it started but when proofed the proof house marked it as per the larges plug gage it would accept.
After 1887 it would likely have been marked as an 11/1 but was still probably actually nearer to a 10. That was a major fault of that system which it took many years to correct by marking actual bore diameter, rather than using a series of plug gages.
