The Workhorse M-LOK compatible rail system was designed to be a lightweight and slimline option to be used on Mil-Spec AR-15 carbines. With M-LOK mounting options at the 3" 6" and 9 o'clock positions and skeletonized areas where M-LOK is not used" this handguard not only offers function but shaves weight where it is not needed. A full length 1913 mil-spec Picatinny rail at the 12 o'clock position has been "notch cut" to match the BAD upper receiver and further reduce weight. Offered in multiple lengths to accommodate different barrel lengths and needs" the Workhorse offers everything you need and nothing you dont. Fits all Mil-Spec AR-15 upper receivers. Inside diameter measures 1.36 ".