Above specs are from Berry's Bullets web site. They do not say that applies to .458 WinMag.
Those 350-grainers must be heavily plated. Might be able to try in the .458 WinMag by starting low and working up, watch what happens to accuracy. Would consider H4895 max load of 80 grains with 350gr bullets as a conservative max baseline, about 2400 fps in your .458 WinMag with 24" barrel. Starting load would be 48 grains H4895 for a higher velocity (1800 fps maybe ?) low pressure starting load. H4198 starting at 40 grains with a filler would be a good starting point too. I would use a foam wad (caulk backer rod) filler with H4198 or H4895.
I am kind of stuck on 400-grainers. Have not used the 350-grainers much except testing to 2800 fps with the 350-gr TSX. H4198 is the powder there, at doses up to 81.0 grains , seated long.
72.0 gr of H4198 is a good 2526 fps MV load for the 350-gr. Hornady RNSP at COL of 3.165" in WW brass with F215 primer. Might try backing down from that with a filler, looking for accuracy at no more than 1950 fps. Maybe around 50 grains with filler.
Ron aka "Rip" for Riflecrank Internationale Permanente NRA Life Benefactor and Beneficiary .458 Winchester Magnum, Magnanimous in Victory THE WALKING DEAD does so remind me of Democrap voters. Donkeypox.
