Glock 40 Gen4 MOS Long Slide Glock 40 gen 4 10mm-glock 40 for sale The G40 Gen4 in MOS Configuration is chambered in 10 mm Auto and combines a long slide and barrel for improved velocity with a high magazine capacity. The Modular Back Strap design and the reversible magazine catch let you instantly customize its grip to adapt to accommodate any hand size. The MOS Configuration makes an easy job of mounting a reflex sight on the pistol’s slide without the need for a new slide or separate mounting system.