The prospect of multiple uppers on a single SBR lower is intriguing...so what else should I consider for my 2nd upper?  (just one pistol until paperwork approved).  Here's the list so far:

-Already have a 6.8 with an 18" ARP barrel and it is the most accurate AR I own, even nosing out a 20" Criterion in 5.56
-No experience with a 300BLK

-Reliability...both should be GTG (long drive to shoot)

-Magazines...plenty of both.

-Ammo...decent stock of 6.8.  Don't own any 300BLK, so yet another round to buy and stock...but it is cheaper to replenish.

-Most shots around 100 yards.

First upper is a 10.5" 5.56...what else should I be thinking about for #2?   (not ready to pop for both :)

If you have a suppressor, I would go for .300blk, as they are really quiet and fun with a subsonic round.  If not, get one of H's 12.5" 6.8s.  Same outstanding accuracy and you already have the ammo for it.