Reckon that would be called an 8 bore.
IIRC, the ones we made were about 1 1/2 at the breech and tapered to 1 1/4 at the muzzle. I will have to check that tomorrow to be sure.
We ran the twist at 1:144 or however close the barrel maker could get to that.
The weight in the barrel was appreciated when it was taken up to a service load of 300 grains of Fg.
Our double rifle used somewhat thinner barrels but used the same powder charge.
I shot .820 ball out of it.
One fellow took a double 8 to Africa and shot a male lion with it. It did a 1 1/2 flip backwards. The ball went all the way through it lengthwise, shattered the pelvic girdle, exited and kept going. Yep, lots of power in that load. Wish I could remember the ballistics of it, but it has been 20+ years since I chronoed it. Offhand it gave a 2" group at 50 yards, one each barrel.
John (Bigsmoke)