A 6x45 would be my first thought since you are limiting yourself to the .373" bolt-face. Another thought would be to do a 223 or 223AI and run the 75, 80, or even 90 grain bullets. You will need a fast twist barrel for it, but it should be do-able. I would go with the Ackley version, just for the added brass life. The extra room for powder couldn't hurt either.
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