There are two allen drive screws used to secure the front sight to the flash suppressor and flash suppressor to the barrel by screw locking the castle nut in place.
The allen drive sizes can vary.
Select a size that best fits the screw head.
Soak the screw in penetrating oil, I like Kroil oil for this purpose, many are rust/carbon frozen. It may take a day ior two of soaking before the penetrating oil gets down through the thread shaft.
Use a short piece of small diameter metal pipe as a cheater bar, slide it over the end of the allen wrench, short end of the wrench in the screw socket, shaft of the wrench and cheater pipe facing right, weapon barrel secured.
Lefty loosey, so turn the screw counter clockwise when looking at the screw from the receiver side for the front sight screw and from the muzzle side when looking at the castle nut securing screw.
If the screw still won't turn free using a moderate amount of force, use a propane torch to apply some heat to the screw and the area surrounding it until they become uncomfortable hot to the touch.
Do this in a ventilated area as the penetrating oil will smoke and the fumes are not healthy.
Quickly reapply the wrench and cheater pipe and the screw should break free.HTH