I currently have two .50 cal longrifles, and both have similar rifling styles, and rates of twist. One has a 42" barrel and the other, 48". The accuracy load that works identically in these rifles is 72 gr. of FFg GOEX, .495" ball, .021" (10 oz) denim pre-cut patches (material washed and dried twice to remove sizing), any lube from mink oil or neatsfoot oil, to LVL or moose milk, to saliva. I use this load for all shooting from 25 yards to 100+. This load shoots flat - level sights - to 75 yards, and requires holding some of the front sight over the rear sight notch for 100 yards.
As far as the calibre choice goes, I like it so much I have two more rifles of that calibre in the 'wings' waiting for time on the work bench. With my rifles in the 9 - 10 pound range, I have no issues with recoil induces flinching, so hold and follow through is comfortable and natural. Follow-through is essential for all shooting, but absolutely crucial for flintlock rifles.
Having said all that, I have to admit that I was surprised to find that the best handling, steadiest rifle I have ever shouldered, was made by my friend Gary Mummery in Kimberly, BC, back in the last century. It was a Hawken rifle with a 28" x 1 1/8" parallel .54 cal barrel, and a 13 1/2" lop. It seemed to be an effort to get the sights to move out of the bull'seye. This is all such fun!