The Magpul, Gen 2 Mag coupler is for PMAG 30 round AR-15/M4 magazines which attach two, 4 rib magazines together for more efficient speed reloads and to keep an additional 30 rounds accessible at all times. This Magpul coupler features a two-piece bolt on, durable and lightweight polymer design. Bolt and nut are both included. Please see photo to ensure that this product will fit your magazines. NOTE: WILL NOT FIT GEN. 3; please see our Gen. 3 Magpul Coupler here.