As quick-targeting reflex sights have made their way onto defensive carry guns, GLOCK developed the G19 Gen4 in MOS configuration. This makes mounting a reflex sight on the pistol’s slide without the need for a new slide or separate mounting System quick and easy. Adding to that, the modular back strap system makes it possible to instantly customize its grip to accommodate any hand size. The reversible magazine catch makes it ideal for left and right-handed shooters. GLOCK 19 in MOS.