The Fortis low profile gas block is designed to work in conjunction with today's match grade barrels. Don't want to mar your barrel installing the gas block? Neither do we! So rock this gas block and put those concerns to bed. The Fortis LP gas block is designed for .750 gas seats and attaches to the barrel with two set screws. The gas block comes with a roll pin and two set screws. FeaturesTwo set screws.750 diameter0.470" Set Screw Spacing