The 458 Win is just a slightly longer 45-70. The same top powders are the same for both. With the 350 grain my favourite powder is 3031. My other favourites in both cartridges for the 400 and 500 grain bullets have been Varget and H or IMR 4895. If you check out the load data, you will find those three powders produce top velocities, but at MUCH lower pressures than most of the other commonly used powders. Even in strong Bolt guns or the Ruger, less pressure at the same or better velocity is a good thing. They also tend to group great.
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