I would suggest shooting cast from the 308. The 300 win mag is a huge case and could pose some problems getting lower charge weights of powders to burn consistently. It may require dacron filler and IMHO that is just a pain in the butt.
PLUS, as you already know, cast rifle bullets are GENERALLY shot at lower velocities than jacketed. I would wager to bet you are looking for something around the 2000-2200 FPS range?
Find out what your bore slugs at and then let me know. Unfortunately it will probably slug at .308, and I don't have a .309 sizer die. BUT, I do have a .308 and .314. .308 would probably work just fine in a .308 bore. Let me know!
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