With my October Country Light American Sporting Rifle, I did some chrono work several years ago. The barrel was 32" long, tapered 1 1/8 to 1" and it was rifled at 1:104. My plinking load was 135 grains of GOEX Ffg, I have lost the paper work on the readings, but I do recall the top readings. I was testing the difference between GOEX Ffg and Pyrodex Select.
At 200 grains of Ffg I got around 1,980 fps with a .610 round ball in Ox Yoke .015 cotton patching on top of a 20 gauge lubricated cushion wad.
Using the same basic loading but with 200 grains of Pyrodex Select, I got 2,004 fps.
Frankly, I was really surprised to get faster velocity with the Pyrodex, but that phenomenon pretty much played out with all my slow twist, big bore rifles, right up to my 8 bore double rifle. My guess is that the wider the bore, the more efficient Pyrodex is. Even with RWS musket cap ignition, hang fires were consistent.
Frankly, I wouldn't advise anyone to try to duplicate this experiment. The only reason I am discussing this right now is to illustrate the amazing performance that can be achieved with a .62 caliber rifle.
John (Bigsmoke)