Frank,
The reason for the 3.8" rifled bore for a James Rifle is a 3.67" smoothbore gun when reamed for a James projectile became a 3.8" rifled bore. Early attmpts at to convert 6 pounder guns to be a rifled field gun involved 106 fieldpieces. seventy-five of these guns were reamed to 3.8" diameter with 15 lands and grooves of a right hand twist. These guns, as a group of 75, became known as James Rifles, Type I. Another group of 31 pieces were machined into a 3.67" rifle because they retained their 3.67" diameter and not the 3.8" diameter of the James version. There have been five differnt groove configurations known to date, such as 7 grooves from Marshal, 8 grovves from Ames and Greenwood, 9 grooves from Greenwood, 10 grooves from Alger and Ames. Most of the guns from Alger lack the marks of acceptance such as the inspectors initials and the U.S. stamp on top of the reinforce, which indicates they were for experimental use only. They also retained their exterior appearance of the 6 pounder field gun. Thus they were also called a 6 pounder rifle.
Ron