Glock 34 Gen4 MOS.

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 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 G21 Gen4 retains the GLOCK accessory rail for your attachments. GLOCK engineers have developed the GLOCK Modular Optic System (MOS) to simplify mounting of popular optical sights without costly custom machining of the pistol’s slide. The GLOCK MOS allows the shooter to mount the sight of their choice, easily and quickly using only a few tools.  The versatility of the system makes changing optics easy.  The shooter is not locked into one optic since switching from one sight to another is as simple as removing the old sight, removing and reinstalling the correct adapter plate, then mounting the new sight. The MOS is offered with five different plate options to comply with the specifications of some of the most trusted optic manufacturers in the industry. The advantages of optical sights mounted using the MOS are increased speed of target acquisition and target transition for faster times in competition, and improved accuracy at both long and short range. Optical sights are popular in competitive shooting and are becoming increasingly popular for concealed carry, home defense and target shooters.