re: Which AR caliber for deer hunting

Infidel,
I'm not a player in the AR game, but I've killed my fair share of deer w/ specialty handguns that gives me some insight. If the 6.5 Grendel doable in the AR 15 platform, I think it would make for a fine whitetail deer hunting gun. I've killed close to 30 head of big game w/ a T/C Contender using a 14" barrel chambered in 6.5 JDJ. My favorite load for the gun used a 120 gr Nosler Ballistic Tip bullet at a muzzle velocity of approx 2450 fps; my most distant kill was 325 yards (although I hit a little back in the paunch and had to make a second shot at 175 yds). 6.5 mm bullets have a well deserved reputation as being very deadly on big game. It looks like similar loads are available for the 6.5 Grendel.

Another good gun would be the 300 Blackout/300 Whisper (or what I have, a 300/221). All these are ballistic twins. My 12" barreled Contender has killed 3 deer, the max distance being 200 yards. A 125 gr Nosler Ballistic Tip at 2200-2300 fps muzzle velocity is a very good deer killer and has negligible recoil. When my son started deer hunting, I set up a Contender carbine barrel for him in 300/221.

I'm sure you'll have lots of good recommendations, but either of the two aforementioned rounds would serve you and your son well.