This Guide Rod Laser fits the GLOCK 20, GLOCK 21 Battery Model: LMS-3x393 (readily avail able silver oxide batteries commonly used in watches) Accuracy (20 Yards): ±2-Inch POA. Laser Operating Temperature: 15°-120° F. Wavelength: 635nm (red-orange) Power Output: 5mW (brightest available - blinks for highest visibility) Spot size: at 7 yds 5/16”, spot size at 25 yds 3/4”. If any firearm has been ported, fluted or compensated either by the original manufacturer or by a gunsmith, LaserMax will NOT guarantee alignment. Custom alignment quotes for firearms may be obtained by calling LaserMax. Please Note:: 1. Expect to change your batteries once a year with typical usage. 2. Battery operating temperature is 15°F-120°F. 3. Does not fit the Gen4 model.