Since it's a factory SBR, I don't believe you can just pin a flash suppressor on a 6921, not with the original lower receiver.
It would still be in the NFA registry.
If you guys hate a 16" barrel that much, do what other's have suggested, buy a 6920 and then get a Colt 14.5" barrel or an entire upper and pin a flash suppressor long enough to make it to 16".
That's a whole lot of expense to go for and only drop 1.5".

