if the FF tube does not cover the gas block DPMS makes a nice 4 rail gas block, good for mounting anything.
Mfr: DPMS
Versatile Platform For Sighting & Lighting Accessories
Four Picatinny rails allow you to mount the right accessories for the task at hand. Lets you attach an iron sight, laser, night vision device and flashlight, all at the same time. Accepts Picatinny or Weaver-style rings. Attaches with dual set screws. Models for standard, .750" diameter barrels and .936" diameter, heavy, match barrels.
SPECS: Aluminum, black, anodized, matte finish. 1¾" (4.5cm) long, 1½" (3.5cm) wide, .750” (19mm) or .936" (23.8mm) I.D.