If you want to run a genuine Bushmaster Dissy upper, most FF handguards will require you to shave down the gas block (which is set up at the same position as the carbine FSB) or replace the gas block with a low profile version from Larue Tactical, PRI, etc. The FSB on a Bushmaster Dissy is not part of the gas system. Also, if you're thinking of a FF handguard you should get the M4 profile Dissy upper, otherwise your rifle will be front heavy. IIRC the lightest FF handguard is the Daniel Defense rifle length. Most popular is probably the Larue Tactical rifle length. Heaviest would be the Troy or Samson rifle length (although to be fair, these last two are easier to install since you don't have to remove the FH, FSB, or gas block - just cut off the delta ring assembly with a Dremel tool).