The Laser Bore Sighter is a tool to set any kind of optics like RedDot or Scopes. The sighter is inserted into the barrel as far as the stop. The laser projects a red dot to a specific point. This one should choose as far as possible to ensure the hits even at a great distance. After that you only need to syncronise the point about a red dot with the point of the laser on the screws of the optics.