Sights on guns are used to adjust for the effects of gravity on a bullet. When you set the sights on a gun, you must set them for the range you are likely to be shooting from. Basically what you are doing is adjusting the "Line from sights" to cross the "Bullet path" at the distance you will likely be shooting from. Beyond this length, your bullet will hit low, closer than this length, your bullet will hit high. This also only sets your gun correctly for level ground. If you shoot down from a hill, your bullet will hit high or if you shoot up a hill (dangerous practice) your bullet will hit low.