Per the Hodgdon 777 FAQ sheet, they recommend a .49 Speer RB and a .02 lubed Ox-yoke patch. At 90 grns you'll be at 1988 fps.
I mention this only because I was in your same shoes. Owned an inline, shot plenty of deer with it then ran across some buckskinners at a Lewis and Clark festival. They looked like they were having fun. So I bought a GPR .54 cal. After rolling 5 deer with it last season, I'm hooked. Round balls are the bomb!
Either way, being a bit spoiled by my inline and it's capabilities, I was determined to duplicate it's performance to the best of the GPR's ability. I bought RB's in three different configurations (different manufactures and sizes) and did the same with patches (from .005-.02, lubed and unlubed). Also tried different powders (Clean shot/American Pioneer and 777, both in FFG). When my testing and patterning was all said and done, it was a recommeded load from 777 that worked the best. 100 gr 777 FFg, .02 Ox-yoke & .53 Hornaday RB. Quality & consistency at 100 yds (off a bench). Now that I know the rifle can do it, I just need to make myself as steady as possible.
One other thing. Get an extra and preferably a synthetic ramroad. Great for the range, perfect for cleanning and unbreakable. :m2c: