The MK1 series, developed by Primary Weapons Systems, is a significant product improvement for the M4/M16-type carbines and rifles. Using the proven long stroke piston system, the MK1 series greatly reduces fouling in the action of the weapon making it as reliable as the AK47 that it was designed after. The MK1 series has been used widely in the Iraq and Afghanistan and has gained wide popularity with security personnel, law enforcement and civilians alike. The origins of the MK1 series began with the fascination in the simplicity and reliability of the AK47 combined with a deep rooted appreciation for the ergonomics of the M4/M16 platform. Having a significant understanding of the deficiencies in the direct gas impingement operating system found on conventional M4/M16-style weapons, PWS developed the MK1 series with the professional operator in mind. Significant product improvements have been made over both the direct impingement operating system as well as previous offerings of piston driven operating systems currently on the market to improve reliability and performance as well as reduce things such as carrier tilt, cam pin wear and carrier bounce. An innovative hand guard system utilizes a proprietary non-free float system that allows the end user or unit armorer to replace individual rails as needed without removing the barrel, barrel nut or even other rails. This system allows the user to attach numerous picatinny rail mounted devices such as sights, lights, and laser aiming devices. The PWS proprietary operating system utilizes an operating rod that is attached to the carrier and a floating head piston that is attached to the operating rod. The MK1 series is built to be true combat carbine. With no unnecessary gas adjustments and only one moving assembly make this the most rugged, simple and versatile piston operating system available. The MK1 series is available as complete carbine or rifle or as a “drop-in” MK1 series upper receiver for use on the lower receivers of existing M4/M16 style lowers. | 
