KY Jon suggests asking the seller to try a 10 gauge shell, or provide measurements.

i have already done so....and was told that a 12 gauge shell was a "perfect fit"....obviously there was no such animal as a 3 1/2" 12 gauge during the era in which the barrels were made. but, as jon states, these guns were built quite heavily and likely would be usable with appropriate 3" loads. marked 4 + 4, so full choked....if they haven't been altered.

best regards,
tom

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland