The two basic types with pics below are
- the cartridge type which is loaded via magazine into the breech.
- the bore type which is slid down the barrel from the muzzle end.
Step 1: Parts and Starting
All you really need is the laser and a mount. I used the el cheapo 1mW red pointer although the more powerful green ones will also work with suitable mounting.
Actually the green ones would be better if you were wanting to sight in longer than 20 metres as well as during daylight hours, it would be more visible than the cheap red 1mW ones.
The mount should be adjustable to compensate for the fact that most lasers aren't centered in their housings and also interface between the laser and barrel bore.
I chose a 19mm bolt as the mount and threaded the end to accommodate different rod sizes. eg BB gun bore is 6mm and CO2 guns 4.5mm (actually 0.177 inch)
I figured I could make use of the ball that the key chain attaches to on the end of the laser, to make a swiveling sort of coupling.