^^^^
Edit: Well there you go, shows what I know.
I believe if your superlight is a postban then you won't be able to put a FH on it because to much material has been removed.
You can verify this by asking bushmaster.
Preban superlights are flared abit at the end to accomidate the threads.
Edit again: This is what a remember reading, from Bushmaster's website.
Note: Post-Ban version will not accept currently manufactured muzzle brakes.I guess this doesn't apply to threading and adding a FH.