A "modern 45-90" is really a Muzzle Loader that uses a cartridge. 500 gr bullets should not work as a correct 45-90 was an express rifle using 3-350 gr bullets.
In my 45-70 Browning High Wall with black or 1000 fps smokeless loads (400 gr Lee plainbase) I just use pure lead as I do in my .452 Whitworth replica muzzle loader with a 450 gr PB Pedersolli mold (aka exact copy of Lyman)
Spg lube and oversize SAECO lube sizing die that only lubes. Either will kill anything that walks in NA.
Suggest you get Mike Venturino's books on BPCR shooting, a wealth of been there done that knowledge.
P.S. if you already have the 45-90 keep it. If you don't buy a 45-70.
