DD DDM4 300S URG 300BLK 10.3" BLACKDANIEL DEFENSE DDM4 300S URG 300BLK 10. 3" BLACK The DDM4 300S AR15 style Upper Receiver Group is designed for those demanding a high performance, short barrel rifle chambered in the versatile 300 Blackout cartridge. Built around a 10. 3 inch barrel, the 300S Upper Receiver Group uses standard 5. 56 bolts and magazines to deliver . 30 caliber projectiles with greater muzzle energy than comparable 5. 56mm platforms. With a wide variety of factory