The factory screw uses an 1/3 Allen head. It has an oversized head, but thin body, which tends to promote over torqing because the size of the Allen key used.
If you do snap the mounting screw as some of us have, we do make an improved bracket/screw retrofit kit for the Matech BUIS
The screw is rated Grade 8 plus, for reference, the gas key on the bolt carrier is rated Grade 8. The factory screw has some thread locker already applied, it's typically good for 3 to 4 screwing/unscrewing then a new drop of thread locker needs to be applied.
CEO, Forward Controls Design