I've had excellent success with both the accuracy and the effectiveness of the Great Plains bullets, in both .50 and .54 caliber. I haven't seen any for sale in awhile and was wondering if they had been discontinued; where do you find them? Never tried them in a scoped gun, but with an aperture sight I can get about a 4-inch group at 100 yards with at least 2 of my rifles and I'm sure my eyesight is the biggest limiting factor.
The Powerbelts shot well for me also, but not quite as accurate as the GP slugs. The only real benefit I can see to them is how much easier they slide down the barrel, way less effort to load than the GP's.
I use only FFg blackpowder, no artificial powders. No heavy charges and no inlines, either. The GP bullets just seem the more appropriate choice to use in a sidehammer traditional gun.