The GLOCK 19 Gen4 in MOS Configuration features reduced dimensions without sacrificing all-important magazine capacity; the modular back strap system makes it possible to instantly customize its grip to accommodate any hand size; reversible magazine catch makes it ideal for left and right handed shooters. The G19 Gen4 in MOS configuration makes mounting a reflex sight on the pistol's slide without the need for a new slide or separate mounting system. MOS pistols come with a cover plate and four different plate options that fit the sights from the most trusted optic manufacturers in the industry.