Right now there are a metric of cal. choices for the AR platform. I have a friend of mine that is using Olymic Arms AR's exclusively on his hunts. I cant remember what the cal was of the one he brought when i hunted with him last but the casing was a big fat body and the projectile was probably in the .243 size.
He said it shoots flat as a board and will pretty much stack rounds on top of each other.
EDIT: Found out it is a .243 SSM