That looks like a T/C Hawkins rifle. That's an excellent rifle you have there. It is 15/16th on the flats. There is no smoothbarrel offered by Green Mountain Barrel Company for that rifle. There are some .32, .40, .50 & .54 caliber barrels that would drop in there in 1:28, 1:48& 1:70 twists depending on what projectile you wanted to shoot.