The Complete MARCK 15 5.56 Weapon System. This MARCK 15, also known as the "Hydra", is the base system for everything associated with our system. This complete weapon system is ready to fire out of the box. It includes the Modular Lower Receiver with a 5.56mm Magazine-Well, the QCB-D Upper Receiver and a 16-inch barrel with a low profile gas block. With this base system, you can purchase and utilize the rest of ever-growing system including our various magazine wells and caliber conversions. Change barrels in seconds and utilize the correct magazine for the caliber you desire. With several magazine wells and caliber conversions already available, and many more coming into production, this weapon system continues to grow and become the most versatile Black Rifle platform the industry has ever seen. Simple as changing just a bolt and barrel on this system the user will be able to fire 7 plus calibers including .22 rimfire, .223 or 5.56, 6.5 grendel, 6.8 spc, .450 thumper, .458 socom, and the .50 beowolf. FFL necessary to purchase. All Federal and State Rules apply.