Sons of Liberty Gun Works M89, M-LOK Handguard, 13, Fits AR Rifles, Anodized Finish, Black M89-13 The revolutionary" patent-pending Drivelock design in collaboration with Icon Defense" uses three independent wedges that interface with the angular grooves in their proprietary barrel nut. This system clamps the rail onto the steel barrel nut with extreme force and precision" while also driving the handguard rearward towards the upper receiver. The slightly larger ID of that extrusion gives you more real estate around the gas block" even more rigidity than the M76" and takes longer to heat.