MARCK-15, also known as the "Hydra", is the base system for everything associated with the MGI® Modular Weapon System. This configuration of the Hydra is setup as a 5.56 x 45. It includes the MGI® Modular Lower Receiver with an AR Magazine Well, an AR Magazine, the MGI® Quick Change Barrel Upper Receiver (QCB-D), a 16-inch 5.56 Barrel with a Low Profile Gas Block, the MGI® Modified Bolt Carrier with a 5.56 Bolt and Firing Pin, the MGI® D-Fender D-Ring for increased reliability, and a Standard M4 Profile Six Position Mil Spec Stock. Everything comes packaged in a Standard Hard-Sided Lockable Pistol Case. With this base rifle, you can purchase and utilize the rest of an ever-growing system including our various magazine wells and caliber conversion kits. To convert this system to another caliber, the most you will need to change is the barrel, bolt, and magazine well to the desired caliber. This gun is like new. Less the 200rnds fired. Looking for something new. Still in the ar field. Just seeing what might be out there. Ffl and background in gresham at aft. Extras on the gun not included.