Is it Uncle Herschel doing the reaching?
Look up Uncle Herschel on Urban Dictionary. Pretty sure there's a code of conduct I'd be violating if I posted it myself.
I've got a 7.5" 5.56 upper from these guys and it's good to go. Can't speak for the 300 uppers but if they're anything like the 5.56, you'll be fine.
http://androcorpind.com/index.php?route=product/category&path=80_102
I agree on keeping it short. My brother has a 16" 300 BO upper and even the subs in that barrel length are right at the limit. Some do go super. UMC sub ammo is a lot more consistent than the Federal AE Sub ammo.