You dont state what bullets you want to use for a reduced load, but if you want to go cast, I say this because I havn't use this powder with jacketed bullets. Accurate Arms XMP 5744 is a powder designed around the Big Sharps cartridges, to be used without fillers. There are loads listed for the .458 Mag. in there manual. Its "the" powder I use when shooting cast in my 2 .416 Rigby's. (Ruger #1 & CZ550) I also use it in my Sharps '75, 40/65 Win. You might want to check it out. CRASH87