This is an off shoot on this topic but related. revdocdrew has stated that the chambers in this gun are likley to be 2 5/8 based on age? I have a grade #3 gun built 1899 and had a similar question regarding chamber sizes. Referring to Brophy's book, the chamber dimensions are given for each of the gauges on plans for the guns. The 12 ga is shown as 2 3/4. When did Hunter change the chamber sizes from the previous standard 12 ga 2 5/8 to the 2 3/4? I know the 16 was near the last to change and happened some time around WWII.