There are definitely limited options in the larger caliber AR platforms, that's largely because they aren't well suited for long range accuracy. There are some small town options that if anyone is interested I can get info for. That's why I suggest making your own with a receiver set/bolt from the brand/caliber of choice.
IMO, the best 30 cal AR's are the LWRC REPR (i have 2) and the Larue predator series. But now we will get caught up in piston vs DI
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Ruger just came out with a wicked new rifle too. I don't love ruger but i'm getting one of these in 308
http://www.ruger-firearms.com/products/precisionRifle/models.html