A 1.3 inch I.D. will accommodate most low-profile gas blocks. Some interference with clamp-on style low profile gas blocks is possible. MLok slots at the 3, 6 and 9 positions are manufactured to MAGPUL standards. This ensures proper fitment with any MAGPUL certified accessories. Every AR15 Cornerstone handguard comes with a pre-heat treated, 2 inch length 4140 steel barrel nut. This ensures rigid, rock-solid accessory mounting. The barrel nut has been nitrided, not phosphated. Mounting instructions and hardware are included with every AR15. Specifications: