21MM Width Rail With Cross Slot - The blue laser is compatible with Glock 17 19 w/ a 21MM width rail, which should have cross slot to tighten the laser sight. Bright & Safe Laser: Class IIIA, <5mW - The output of the blue beam is <5mW, Class IIIA. The blue pistol laser is very safe and highly visible in any environments, which can reach up to 50-100 ft. during the day and up to 600-1000 ft. at night. Left & Right Switches - Easy ambidextrous activation for left and right-handed users. The blue gun laser is made of materials, which offers shockproof and stable experience. Rechargeable Built-in Battery - The blue beams for guns is powered by a built-in rechargeable battery, A full charging takes about 1.5 hour and provides about 3-4 hours max running time. Kindly Be Noted - Please slowly tune Elevation and Windage interactively, only one direction may lock or damage the threads.