Well if finding and affording ammo was not an issue, then the 6.8 or 6.5 Grendel in a piston AR (or after/if it's out and proven itself maybe the Masada/ACR). If there are no feeding issues with it (thought I heard about some once) then I think the Grendel is probably the better of the two. Recoil is significantly less than .308 but it retains its energy just as far if not further and it outperforms 5.56 terminally with a slight tradeoff in recoil and ammo weight. I would think (don't know for sure) it would be able to handle game on up to deer and probably a little larger.
Since rifle was specified I didn't say 12ga, but a Mossy 500 pump 20" would be my go to if I had to pick just one. You have a plethora of options for ammo types so it's task adaptable with just a quick change of ammo and within it's range a 12ga can drop nearly any creature on the planet (nearly, elephant, cape buffalo? ... idk).
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Last edited by Tucker 1371; July 1, 2009 at 07:16 PM.