It's called a raefdginner. There are different types, and most of them are quite expensive. Many good rifle scopes have tick marks on the reticle to use as range finders. Each of the marks represents a certain width in inches at any given range. Military versions are based loosely on the width of an average man's shoulders, or on the height of an average man.It's best to check the manual that comes with the scope to find what works best.