Re: Bushmaster Varminter .223, want .243 upper & barrel for deer any recommendations?
I'll second the 6.8 SPC. It will drop right on your .223 lower. I don't know about smaller towns but ammo can be had at places like Bass Pro Shops or Cabelas. I have a Rock River Arms upper on order for deer hunting right now. Figure on putting a red dot sight on it for still hunting and deer drives. Even out of a 16" barrel it has fairly impressive ballistics on the AR platform. The 6.5 Grendel is a proprietary round and really suffers when used with a short barrel. Unless you are talking long range shooting, it offers nothing that the 6.8 SPC doesn't. (I would like to have one with about a 22" barrel though, but kind of pricy.)
The 7.62x39 is certainly another option. Not quite as flat shooting but would be great on deer at moderate range.