The 6390RDS holster is specifically designed for firearms with red dot sights mounted to them. This open-top holster features the Automatic Locking System (ALS), an internal locking device that retains the weapon in all directions with no straps or snaps to manipulate. Once the weapon is holstered, it locks into place. The holster is completely operable with the thumb and once the ALS is deactivated a simple straight-up draw is used. Features a mid-ride belt loop that is rides close to the body. Safariland cannot guarantee that RDS holsters will be compatible with non-factory (Glock MOS, M&P CORE, etc.) mounted red dots. Numerous variables including depth of RDS into slide, use of dovetail, placement on slide, etc. Can all affect the fit of the holster.