Clarification Please: I have a pre-ban Colt AR-15 (R6420) with 16" barrel. Considering a pre-ban flat top barrel assembly at 11.5". Does the pre-ban status exempt the sub-16" barrel from the NFA/SBR designation? As in, legal to own in MA? Much obliged, James.
Not if you like your doggo. To add to @Shawnyc options you could pin and weld a muzzle device to get you over the 16 inch minimum. If suppressors are legal where you reside they can be pin and welded too.
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.