Mine had only the right barrel rifled and it was .705" groove to groove.

For my gun, the original ammo for these, was black powder with round ball in a 16 gauge brass case, while the other chamber, exactly the same size, was for 2.5" 16 gauge paper cases.

In my gun, the left barrel was modified choke, 16 gauge.

I used to load brass cases for both barrels.

Incidently, the chambers were only 1/8" short of standard 2.3/4" plastic hulls. My gun shot beautiful full patterns out of the left 16 bore smooth barrel, and did not do-nut pattern from the cylinder straight rifled tube with 2 3/4" 1 ounce of # 7 1/2 lead field loads.

The straight rifling handled shot nicely on rising grouse - quite deadly, actually.

I slugged the bore with a .714" ball to get full depth, and then tried the normal .682" ball I use in my .69 ML rifle. These engraved a bit and are what I used to test accuracy in the target below. Mine had a .675" bore, almost a true 15 bore (.677"), bore. I also used 2F and think it might group better than 3F, although 3f will give higher velocity.

I only targeted mine at 28 yards.

I refinished the stock.

It patterned even better with black powder and normal card and fiber wads. I used 14 bore wads for a perfect fit in the 16 bore brass.

They are really neat guns, Bob. Wish mine had been rifle both tubes as yours is.