I cast a 500 grain spire point, paper patched w/ 9lb. onionskin to .458. I also shoot 500 grain round nose Postell-type bullets, with 4 grooves.
I load these on 115 grains of Goex 1F, .030 veg. wad and .600 compression. A big grease cookie (almost 1/8") to keep fouling soft in that long barrel. Another .030 veg. wad. Bullets are seated by hand.
By the way, I do NOT size my cases anymore. Treat the loaded rounds gently, and they'll be fine.
The rifle is a .45-120 Sharps, made by C. Sharps; 30" heavy octagon barrel and LR sights.
Hiding in plain sight...