http://catotheamerican.wordpress.com/2011/06/22/praxis-using-a-duplex-reticle-as-a-rangefinder/
Praxis: Using a duplex reticle as a rangefinder
Many American households have a scoped deer rifle in them. Common calibers are .30-06, .308, .270, .243, and a variety of sporterized military surplus rifles from around the world. All of these rifles are capable as being used for “designated marksman” functions. Let us assume that the excrement has hit the rotary oscillator, and you are now in some sort of leadership role of a rag-tag group of people. You find that you have a few SKS and Kalashnikov variants, an AR-15 or two, and several scoped deer rifles. What to do from here.
You AK/SKSs are going to function as your close-quarter and small range battle rifles. With the sights that both of these types of rifles have stock, you will probably be using them inside 150 yards. Your ARs (if A2 sights) should have a battle sight zero of 300 yards. You scoped deer rifles will operate between 300-500 yards.
Although your ARs are good for this range too, your shooters might not be. (Have you gone to one of those marksmanship clinics yet?) Most scoped deer rifles will have a duplex reticle, meaning one which is thick on the outside, and becomes a thin line towards the intersection of the crosshairs. And this reticle will become your rangefinder, but if you have another reticle, the process is the same, if the specifics different.
First, we need to go over something that is unfortunately like math. When we talk about a group of shots, you can tell someone you shot a 2 inch group, but unless they know the distance, it’s a meaningless statement. For simplicity’s sake, we’ll be using meters and yards as almost interchangeable. I know they’re not, but this system uses both, please just roll with it.
For accuracy, 2 inches at 25 yards is nothing to write home about, but, 2 inches at 100 is better than we need at this stage of the game. To clear this up, there is a system called Minutes of Angle, or arcminutes. A minute of angle (MOA) is 1/60 of 1 degree of a circle, which you’ll remember has 360. This works out to about 1 inch per 100 yards of distance. This allows us to discuss accuracy regardless of distances involved, since MOA is a ratio, or angle. A 2-minute group at 25-meters is 0.5-inches, and a 2-minute group at 100-yards is 2-inches; and both are equally accurate; the take home bit is, they are 2moa-accurate.
You will need to print out one of the M16 25m zero targets . Each of the squares on this sheet is a quarter of an inch. This is crucial, as a 1/4-inch at 25-meters is exactly one minute of angle.
Post your target at 25-meters (82-feet), and build a good prone position. Use your rifle sling to help support the rifle. If you don’t know how to do this, get thee to a marksmanship and history clinic! For this, a fixed-power scope is best, but if you have an adjustable power scope, then dial it down to the lowest power setting. Remember, higher power doesn’t mean more accurate, as it will amplify all of your movements as well.
Get down on the rifle, and place your reticle on the center of the target. Now you need to take note of how many squares are taken up on the vertical crosshair between the place where the duplex reticle becomes small, and the intersection of the crosshairs. Write this down. It might be something like: 8moa. Now, see the thick part of the reticle, see how many squares it is wide. It might be 2moa, or less. Now, see how wide the thick part of the horizontal cross hair is in minutes of angle, it might be 12moa. Of course, since the reticle is in a circle what goes for the vertical holds for the horizontal and vice-versa. Use whichever is more natural for your target’s shape.
Now, if you know that 1moa is 1-inch per 100-yards, and you know the dimensions of various objects, you can determine distance. From shoulder to shoulder the average man is 20-inches wide. From cheek bone to cheek bone, he is about 8-inches wide. Go measure how tall the average sedan is, or SUV, or door on a house, or garage door. All of these can be used with minutes of angle to figure out distances.
If we know that a human shoulder-span is about 20-inches, and through our hunting scope we seek that he takes up half of the small line of a cross hair, which we measured to be 8moa, we know he is 4 minutes wide. 20/4=5, so 500 yards. He’s at the extreme range of our deer-rifles. But, we’ve been to a few marksmanship classes, and we practice an awful lot; we know we can put five shots in a 4-minute group.
If he’s friendly, he’ll be glad to know he’s got a sharp eye watching out for him. If he’s not, well, we might be forced to ruin his day.
But you need to get to a 25m (82-ft) range, or set one up. Since you won’t be shooting, you can do this in your backyard. Before you do, make sure the rifle is clear and safe, and the ammunition/mags are nowhere near you, use of a chamber flag would not be overkill. Once you figure out how to use your standard duplex reticle as a rangefinder, you can got from shooting minutes-of-deer to minutes of angle, and you might save the lives of your tribe as a designated marksman.
The take-home part of this is that 1moa is 1-inch PER 100-yards. So, 2-inches at 200, 3.75-inches at 375-yards, and 10-inches at 1000-yards. Focus your shooting at 4-minutes, and you will hit a man-size target a 500 yards. Firearms skills build rust fast, so you might have the most tarted-up rifle on the block, but if you can’t use it, it’s just so-much dead weight. Even the lowly deer-rifle will out-shoot you. Get to a clinic, and practice.
The half-hour you spend doing this now could be invaluable in a crisis.
Long live the Republic,
- Cato, the American.
