So your 1:66 twist is perfect for round ball use. You didn't say which ignition system your rifle is, but I will asume it is percussion. I like 90 grains of Goex 3f for 50 cal loose powder charges in 50cal rifles. I would also offer that I have found that .010 thick pre-lubed patches work well for most round ball rifle barrels. If you find your load seems to load too loose and accuracy suffers then you have two options, try a thicker patch material, .015-.017 may work or try some 495 round balls with the .010 patch. This load will harvest deer sized animals as long as the shooter puts it in the vital area.