.450 bushmaster is 1.7in OAL so its deer legal for 2012. i would go with whatever ammo is more available since you dont reload. if you did reload the wildcat .35/25 WSSM is very tempting to me but it is harder to find a barrel for it.
6.5 Grendel.
.308 ballistics in a round that fits an AR-15 lower and is hunting legal. Ammo was just accepted as a standard SAAMI round so more companies will be producing it (thus lowering cost). Unbelievably accurate out to 1,000 yds, it can be configured with a 16" barrel to make a lightweight packin' rifle with a lot of punch. 20" are popular as well.
Lapua Scenar bullets have one of the highest ballistic co-efficients you'll ever see.
Besides, someday maybe you'll want to try a little open country deer hunting or long range target shooting and the Grendel gets it done.
I voted 3oo blackout, but real depends on what he likes to do plinker, hunter, long ranger. I have a 458 and fire big game its the cats meow, but ammo is expensive. The 450/Beowulf seem to be more readily available and more inexpensive. I just purchased an Ak74 to plink, I would like to shoot in am AR15 to compare. Long range go Grendel...just my2.
I have a 458 and a 5.56 so I voted for the 300(BLK) Yeah, 458 ammo is expensive but I do reload, so it's not a issue for me. But it still a fun gun to shoot and you could go up to 500 gr bullets to take care of some very big games if needed, even bigfoot would go down if got shot by one.