Gun, using lasers for distance.
 

So If you have a quad rail on a gun, mount maybe a red laser on the right, a
green laser on the left, sight in the scope at perhaps 150 yards. Then
adjust both lasers to the same point at 150 yards. Seems you could estimate
yardage by the spacing of the laser dots and your under if the red dot is on
the right, over if the green dot is on the right. Of course it might take a
pretty good laser to be visible at the distance you'd want to use it for.

RogerN