7.62 x 51! I'll never go back.
I've got a 9mm AR15 and its a hoot to shoot. But surprisingly it delivers more recoil than you might guess. Its a cheap to shoot round, cycles reliably, but really isn't good for anything. Can't really hunt with it, its OK for QCB but there are better rounds. Seems to me it is an answer to a question that nobody asked. Still, I like it. I just don't know why.If i owned an AR I would want either 7.62x39 or 9mm.
Thanks for all the replies! I guess I'm looking at having an AR that will do it all....provide for food, protection etc. with an easily found ammo.
I think it would be a good idea to have one for SHTF senarios as well as I think the general value of it will increase as time progresses.
5.56x45mm if you can only have less than 10 rifles.
Then you might get one in 7.62x51mm NATO.
The 6.X mm versions have some merit, but ammo availability is always going to be a concern, even if you load your own. 5.56 may be expensive now, but you can get it just about anywhere.
5.56x45 is cheap now but virtually 100% of the supply is Russian made. None of the 'western' ammo makers load that round.