Using a cheap laser pointer to create a laser boresighter, which can be used to check sights on airsoft/rifle/pistol/bb gun and even a crossbow.

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.

The easier of the two versions to make is the bore insert which Ive decided to tackle. This is the way Ive gone about it but other methods will also work.

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.

Step 2: Laser Swivel

Next add the adjustable tabs, determining how long they need to be, mine were 35mm long.
The size bolt was 19mm across the flats which seemed to afford enough space to adjust the laser body.

I also read about a treatment with steel that gives it a black color and experimented with everything steel related. I heated the bolt red hot and then dunked it in old used motor oil, gave the little metal strips the same treatment.

The tips should be pointed to make adjusting easier, works better than the flat bolt end on the rounded laser body

Step 3: Calibrating the Unit

It is a simple matter to calibrate by rotating the unit on bearings with your fingers. In 5min I had it adjusted over a distance of 10m, the laser dot rotating about itself, and not swinging around a center in a circle.
The bearings came from a hard drive platter motor they are 1350zz and the shaft is a tight fit.
I used a hard wood for the bearing base but a folded sheet metal stand would also work.

Step 4: In Use

If you want to use the unit with rifled barrels it would be a good idea to use softer metal like brass or aluminum for the rod, here on my smooth bore BB gun it doesn't matter.