He will probably need Large Rifle Magnum primers as well. I learned this lesson the hard when I got my 7mm remington magnum. Otherwise, I would reference several reloading manuals and see what their accuracy load is. If you have a reliable chronograph, you could also do a ladder test. There are a lot of good slow burning powders that 300 win mag likes. Look at the bullet weight that you're going to use, and see what powders are best for it.
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