Regarding 11.5", you can make it legal with a permanently attached muzzle brake, but you really ought to look at the ballistics. 5.56x45 performance takes a huge hit with shorter barrels. Even the 14.5" barrels lose effective range noticeably. If you have a pre-ban lower, you might consider a 16" barrel. Any muzzle device (other than a can, that's a different topic) is legal. Your barrel has to be at lest 16" long anyway. There are relatively effective muzzle devices that are going to add only ~1.25/1.5" and the gain in velocity and therefore effective range is considerable. YMMV but I think it's worth thinking through.