All you need to do is swap bolt heads, really simple. As far as cartridges go any of the popular 6.5s will get you as far as you want to go, duplicate the trajectory of the 300 and use a whole lot less powder to do it with no need to change bolt heads. 260 Remington 6.5 creedmoor 6.5-06 or 6.5-284 all work great for long range.
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