Mike - I wasn't 100% confident I interpreted your question correctly, but I am taking you to mean what is the diameter of the barrel right at the breech face where the case head's rim would rest. If that is what you meant, the answer is 1.365" to 1.374".
Here are some additional measurements:
- at the breech, total width across both barrels is 2.592"
- height of right barrel at breech from flats to 12 o'clock: 1.365"
- height of left barrel at breech at same point: 1.374"
- barrel thickness at breech, measured just inside the cut for rim: 0.243"
- diameter of rim relief cut: 0.936"
- diameter of chamber just inside rim: 0.862"
- width across both barrels at muzzle: 1.874"
- height (diameter) of barrels at muzzle: 0.961"
- thickness of barrels at muzzle: 0.085"
The barrels are 30" long and weigh 4.95 lbs.