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.