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If you want a 300 yd magnum maybe a 223 wssm
A 6.5 grendel has 1000ft lb at 300 maybe even a 762x39 or 223 shooting some 70 something gr pills

And that would be more like a 600yd capable round wouldn't it?

If you are only thinking 300 just about army short action round in 6.5 or 7mmm will get you there. Any of the 08 cases would be fine. As much as I don't really like it the creedmore would probably be...