Back before laser sights were commercially available for weapons, I purchased a LED laser pointer from Edmund Science and modified it to fit inside a small stainless steel sleeve with adjustment screws. I attached the sleeve to the side of a Crossman .22 Cal. air pistol and modified the pistol's handgrip to hold a standard 9 volt battery and a microswitch. The microswitch lever can be seen protruding just slightly out of the handgrip thumbrest. The laser is activated whenever you grip the pistol and place your thumb on the thumbrest. You can't actually see the laser beam from the side (except in smoke or fog)... I took a little artistic license with this picture and added in the beam.
This pistol is a lot of fun when used in combination with exploding .22 Cal. air pellets.
I must have had too much spare time on my hands, because I also made this carrier box for the pistol.