A laserinside a Picatinny rail! You can mount it on pistols, carbines, rifles, shotguns, full autos, night vision gear, binoculars-- anything with a Picatinny or Weaver rail.
A note about batteries:
1. Expect to change your batteries once a year with typical usage.
2. Cold weather batteries are available for some models.
3. Battery operating temperature is 15°F-120°F