Whenever we run a novice student through a carbine course, one of the first things we teach them after the basic saftey and shooting techniques is barrel sight offset.   Since so many people are using AR15 type rifles, there is a considerable distance between the place where a scope points its crosshairs and where the bullet is going to impact at close ranges.   Something like a Trijicon ACOG gives you a good idea of what people mostly use and we show crosshairs pointing at a dot and where the bullet hits approx 2 inches below that.   This really only has a purpose in showing a shooter what could be applied is using a firearm in self defense in a hostage type situation.

Since lasers have been improving and prices are coming down, we see them showing up more and more on handguns.   While I suggest people take a look at the price they are paying for them and do a lot of research on the durability of the product, a whole new can of worms emerges when sighting in your crimson trace or lasermax laser sights.   Depending on how your laser is mounted, there will be a windage offset that needs to be known.  Crimson Trace laser grips on revolvers can project from right above the grip area or there is a newer style that projects next to where the barrel line is.   Each have their pros and cons, but there is still an offset.   With new tools, there are always slight changes that need to be made to insure an operator knows that with distance, bullet impacts change.