The FastFire 3 is the best selling red dot sight from Burris. It is available with a 3 MOA or 8 MOA dot, and with or without a picatinny mount. Light and tough, the FastFire 3 series puts an unmistakable, bright red dot on your target to give you an immediate speed and accuracy advantage. Can be used as a primary sight or paired on top of an existing sight. Automatic brightness sensor adjusts brightness to match the environmental conditions; also has 3 manual brightness settings Battery access is conveniently located on the top of the sight; automatic time-out feature after 8 hours helps battery last up to 5 years Available with Picatinny/Weaver mount for mounting on rifles, handguns and tactical shotguns DOT SIZE: Generally speaking, the 8 MOA dot is most popular for short range shooters, especially with handguns or shotguns. The larger dot makes target acquisition quicker. The 3 MOA dot is most popular with rifles, or anyone wanting more pin-point accuracy, as the smaller dot covers far less of the overall target area. TECH NOTES: The FastFire 3 will easily work with the Glock MOS system. The FastFire 3 will work with virtually every "optics ready" handgun. However, the Burris supplied mounting screws may not be the correct length for every single handgun. Please contact the firearm manufacturer for specific screw lengths necessary for correct mounting.