Glock engineers have developed the Glock Modular Optic System to simplify mounting of popular optic sights without costly custom machining of the pistol’s slide. Optical sights are popular in competitive shooting and are becoming increasingly popular for concealed carry, home defense and target shooters.The GLOCK 17 Gen4 MOS, in 9×19, introduces revolutionary design changes to the world’s most popular pistol. The Modular Back Strap design lets you instantly customize its grip to adapt to an individual shooter’s hand size. The surface of the frame employs the new scientifically designed, real-world-tested, Gen4 rough textured technology. Internally, the new GLOCK dual recoil spring assembly substantially increases the life of the system. A reversible enlarged magazine catch, changeable in seconds, accommodates left or right-handed operators. The G17 Gen4 9x19mm pistol also retains the GLOCK accessory rail for your attachments.Your new Glock 17 Gen4 MOS pistol is delivered as a factory new, complete set. The hard shell case which houses your pistol also includes a speed loader, three magazines, two backstraps, a cleaning set, a MBS tool, a cable lock and instruction manual.