This is a question that comes up in a variety of different ways and I’ve run across it at the range as well. The reader was not sure if he could range a target using a duplex reticle. The duplex reticle was not designed as a ranging reticle. To range across a wide set of situations you need a reticle graduated in fixed divisions across the horizontal and vertical stadia. There are a number of proprietary systems available to the tactical shooter but the grand daddy of them all is the U.S. Army Mil-Dot reticle (shown below)
However, you can also get a fairly decent range estimation using a duplex reticle.
The duplex reticle assumes an 18” target and a variable power scope for example 3.5 x 12.
To range with a duplex reticle, identify an 18” target then adjust your magnification on the scope to bracket the 18” target between opposing heavy posts as show below.
In this sight picture I used the target width of 18 inches and bracketed the target between the horizontal posts. Once you’ve done that look at your magnification setting and multiply by 100 to get an estimate of your range. With some practice, you can perform this very quickly and achieve an accuracy of + or – 20 yards. You can also interpolate for 36 inch targets or 9 inch targets.
Corrected: 27 Aug 2014
We try to be helpful but at times we don’t succeed and this is one of those times. Ranging with the duplex reticle is based on a 16″ target not 18″. You also need to do a little legwork the next time you go to the range. Here’s what that means.
When you look at your magnification adjustment ring, some manufacturers, Leupold for example, will etch magnification on one side of the ring and yards on the opposite side of the ring. If that is the case, bracket the 16″ target using either the vertical or horizontal stadia, then read the range markings for your range estimate.
If you do not have range markings on your magnification adjustment ring, take a 16″ wide target and run it out in 100 or 50 yd ( send your buddy down range but make sure your weapon is safe) increments then note at what power setting you bracket the 16″ target, and write down the range. This will allow you to correlate magnification setting to range. You can then memorize that data or make up a crib sheet.