you didnt list what type of pistol your shooting so I'm just going to try to guess at some probable loads,if you have access to a chronograph it would help immensely.
the listed load data for 454 casull using h110 and a 300 grain jhp is 30 grains.I reload 50 ae and my loads max out at 32 grains of h110 using 325 grain speer jhp's.
using cast bullets you would usually reduce the powder charge because pressure would increase using a heavier bullet.
looking at the 475 linbaugh and the 500 linbaugh they list loads for 380 grain bullets at 30 grains for the 475 and 37 grains for the 500.
I would use 30 grains and chronograph the rounds to get an idea of your velocities.
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