Fortunately, some companies like Freedom Ordnance march to the beat of a different drum and invest their creative energy into innovative, unique products like the FM-9D. The FM-9D is the successor to Freedom Ordnance’s belt fed FM-9 upper receiver that fit AR-15/M16 lower receivers and was made from late 2015-2018. Pistol, Rifle, SBR or Upper ReceiverThe D Model is sold as a pistol, rifle, SBR or an upper receiver alone. So, you’ll need to change to a slotted bolt if you intend to use the upper receiver from a complete FM-9D on an AR-15 lower. Freedom Ordnance also makes dedicated mounts that attach to the top cover for the Trijicon RMR and other optics.