A comprehensive resource for safe and responsible laser use
Laser gun sights
Small diode lasers, using essentially the same technology as laser pointers, are frequently used on firearms. A laser is placed on the weapon to emit a beam parallel to the barrel.
The visible laser light can warn a suspect that he or she is being targeted by a weapon; this may diffuse a threat. In other situations, using a non-visible infrared laser along with night vision devices allows tactical teams to see the laser dot, without a suspect knowing that he or she is being targeted.
Seeing the laser dot’s location can help with aiming the weapon, but does not necessarily indicate exactly where the bullet will terminate. There are other factors involved such as bullet drop, windage, the laser not being exactly on the same axis as the barrel (otherwise it would be in the barrel), and the bullet’s slower-than-light time of flight.
A good overview about when laser sights are appropriate, and what types are available, is in the article “Got a laser in your sights?” by John Marrs, published at Officer.com in May 2008.