Groove diameter should be .690, but with muzzleloaders, the bore diameter determines the gauge. In this case, it has a .690" bore, with .012" deep rifling, making it actually a 13 bore to the bottom of the grooves, at .714".
: In Forsyth's day, a groove diameter of .005" and 104" of twist in a 24 to 26" set of tubes (double ML rifle) would make it .700" groove diameter. A .675" ball would require about a .018" patch, whereas I use a .684" ball with .018" patch in my deeper grooves.
: A very recent test put a 3 shot group at 100 meters, shooting off my elbows, into 1" X 7/8" group on centres using the Express sights. We made the rifle in 1986 using a Green River River Works barrel.
: That .500 3" BPE was a popular one for India. A close friend has one such double rifle marked on the rib, in gold script & in Italics, Made of Maharaja ????????????. I think it is a John Blisset, but haven't seen the rifle for many years - memories, you know. It shot splendidly and together with Lyman's, .515 conical 450gr. bullet and around 130gr. r 140gr. Pyrodex. I remember shooting 2 1/2" with 2 lefts and 2 rights at 100 yards.(before change to metric system)