Practical Rangefinding

Practical Rangefinding

How do you determine the distance to a remote object without using a rangefinder? Most riflescope reticles are crosshairs, usually thick at the edges, thin where they cross. You can determine what the width and height of the thin part of the crosshairs are with a little bit of measurement, and a little math.

Take your riflescope, and position it in the largest room of your house (because it may not focus close enough in the smaller rooms). Find an object that exactly fills 1/2 of the thin part of the crosshairs (the space between the cross and where the crosshair gets thick). Measure the distance from the scope to the object, and the diameter of the object. The 1/2 of the thin part of the crosshairs subtends an angle calculated by:

degrees = 57.296 * object diameter / distance

Make sure that your units for object diameter and distance are the same! Let's say that it is 25 feet from scope to object, and the object is 1 inch in diameter. The math tells us that the space between the cross and the thick part of the crosshair is 11.45 minutes of arc. Almost certainly, it is actually a 10 minute of arc space--you just haven't measured all that accurately.

How much is 10 minutes of arc? By a happy coincidence, one minute of arc (moa) is almost exactly one inch at 100 yards. (One inch at 100 yards is actually 0.955 moa, but that's close enough that for ballistics below 500 yards we can treat them as identical--the inherent uncertainties of bullet weight and powder variations will outweigh these minor errors.)

If you need to figure out the distance to a remote target, you now have a measuring stick. Most game animals are about 18" across the brisket. (By coincidence, a soldier is also about 18" across the torso.) If a deer's brisket fills the thin part of the crosshairs, but nothing outside of the crosshairs, it is slightly more than 100 yards away. If it fills only half the space--so from the cross to where the hair becomes thick--it is twice that distance, or slightly more than 200 yards away.

The same technique is used by snipers and hunters when working at unknown distances. The standard exterior door is 36" wide, and 80" tall. If a door frame vertically fits into the thin part of the crosshairs (which in our example is 20 moa), that means that it is 80"/20" * 100 yards--or 400 yards away. If it just fits between the cross and the thick part of the crosshairs, it is 80"/10" * 100 yards--or 800 yards away.

I mentioned earlier that 1 inch at 100 yards isn't exactly 1 moa; it's actually .955 moa. How far off does this mess up rangefinding at longer distances? At 400 yards, that 80" doorframe is actually 19.10 moa, not 20 moa--so if you use these formulae to figure out the distance, you are likely going to be off by about 4.5%. You estimate, because it fills a 20 moa spot in your scope, that it is 400 yards--but it is actually 383 yards. This is not a huge difference, but it does mean that the bullet is going to arrive several inches higher than your point of aim.

This discrepancy becomes more and more important above 500 yards, because most bullets, by this point, are falling very rapidly under the influence of gravity, so a 4.5% error in range will result in a bullet hitting many inches above the aimpoint. There are people who go after elk and other game animals at these kind of distances--but if so, they need to be prepared to think of that 10 moa "measuring stick" on their reticle as 9.6" at 100 yards, 48" at 500 yards, and 96" at 1000 yards, instead of the easier to calculate, 10", 50", and 100", respectively.