Hi,
I use a 0.648 ball and about 55 grains of 3F Swiss powder. That powder charge fills he chamber completely. It shoots very accurately out to 100 yards (furthest distance I've shot it). I experimented with carbine ball (0.615) and about 60 grains of powder but could not keep a group within an 8" circle at 50 yards from a bench. I suspect the Rifle Shoppe barrel and action are not exact copies of the originals with respect to the inner barrel dimensions and the powder chamber. As best I can calculate from Ferguson's patent drawings in Bailey's book, the powder chamber should hold close to 70 grains of powder. I can only get about 60 grains in mine behind a 0.615 ball and 55 grains behind a 0.648 ball.
I follow Bryan Brown and Rick Roberts recommendations for lubing the balls and breech screw but their powder and ball recommendations do not work at all in my rifle.
Elk Hunter, I suspect a 0.642 ball might work well and urge you to try it. One other issue to mention, powder fouling is only a problem for the screw plug threads if much accumulates in the threads above the chamber. That is because it is those last threads that really tighten down to seal the breech. If you over fill the powder chamber, screw the plug closed and then sweep the excess powder in to the pan as priming, you won't get many shots before needing to clean the plug. However, if you carefully load the chamber so no excess powder clogs the upper threads, as I do, you will get a lot of shots off without needing to clean the breech. I usually get 25-35 probably depending on temperature and humidity. With 3F Swiss powder, fouling in the barrel and chamber does build up much and does not appear to affect accuracy. Also, I prefer longer flints between 3/4"-1" long because the lock works best using them. The flint cock does not over hang the pan very much so you need longer flints to reliably knock the frizzen out of the way.
I shoot my Ferguson rifle a lot and really enjoy it. I bring it often to our informal woods walks and I do well with it. However, my experience shows me that the rifle was no "super weapon" that could have "changed the outcome of the war". I suspect if many more Fergusons were issued the result would have been many more Fergusons out of commission with broken stocks and lost breech plugs.
dave