The usefulness of a spotting scope to rifle shooters is undisputed, because they are the primary method we have of observing animals at long distance and of spotting bullet hits at extended ranges. But did you know the usefulness of a spotting scope can be increased substantially by putting a ruler inside it?
And no, you can't do this yourself. What I mean is upgrading your current spotter to a model with a built-in ruler.
A ruler inside a spotting scope is nothing more than a measuring reticle placed inside the scope and calibrated so the hash marks subtend a specific and known distance. While I suppose this feature could be used in measuring the size of some animal—or perhaps even their horns—its primary usefulness is in measuring the distance of a bullet's impact from the intended target. Not that we want to know how many feet or meters the hit is away from the target, but rather we want to know how many minutes-of-angle or radians the hit is from the intended impact point.
Measuring that distance using a spotting scope's reticle allows the shooter to dial that measured adjustment into his rifle scope for what should be a second-round hit. So, in practice, a spotter instead of telling a rifleman to hold 15 inches to the left, can simply measure the distance of the miss using the spotting scope's reticle and then tell the shooter to dial in a specific correction. At 500 yards that correction would be 3 minutes-of-angle.
Of course, using this system requires both the spotting scope and the rifle scope be set up for the same unit of measure—minutes in the imperial system, or radians in the metric system.
Spotting scopes with this feature aren't commonplace yet, but manufacturers are slowly adding them to their line-up. The photo I've attached was taken through a Leupold Mark 4 spotting scope, and it works perfectly. Vortex puts the ranging reticle of their Razor HD scope into a removable eyepiece, so the user can switch between minutes and radians for ultimate flexibility.And another option is provided by Swarovski and their STR-80 spotting scope. It incorporates a "switch" that allows the user to turn the reticle on or off depending on need.
As long-range and precision shooting continues to grow in popularity, expect to see more manufacturers offering these products. The usefulness of a ruler inside a spotting scope certainly warrants it.
After all, measuring is always more accurate than guessing.