My .40 was only for target shooting and for the squirrel rifle match at Hefley Creek. The barrel was a Goodoien "match" barrel with a .398" bore and .418" groove diameter.
I used a couple different balls in it, a Lyman .395" mould that cast .398" balls with .021" to .0235" patches. I also used a .400" ball with the same patches and this load combination
showed slightly better accuracy at 50 yards. At that time, my best loads were shooting 1/2" and slightly smaller, 5-shot groups at that range, from a bench-rest.
My powder charges in 3F and 2F GOEX varied depending on the lube I used. With a water based lube, either WWWF + a tich of neetsfoot oil, or just spit, I used 62gr. 3F or 70gr. 2F
to get identical accuracy. With a slippery lube like MR. Flintlock or LeeHighValley lube, I had to use 65gr. 3F, or 75gr. 2F to get the same tight shooting accuracy at 50yards.
Incidentally, both of these loads produced 2,240fps, ie: the 65gr. 3F and 75gr. 2F, when using either the .398" ball or .400" ball. A short starter is necessary to start these loads but
the rifle's 3/8" hickory rod was all that was needed to seat it, with 2 or 3 fingers on the rod.
The smoothed crown on the muzzle is necessary for starting these snug loads. A long coned muzzle makes starting these combinations most difficult.