Author Topic: Sizing Up Your Sights - How and Why (October, 2012 Pilot)  (Read 2042 times)

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Sizing Up Your Sights - How and Why (October, 2012 Pilot)
« on: October 19, 2012, 01:31:01 PM »
Suppose someone told you they had a gadget that would let you sight in your rifle without walking downrange, would help compensate for cross-winds, and allow you to estimate the distance of targets at unknown ranges.  Surely this whizzy piece of gear would cost several hundred bucks!

Actually, the cost is $0 because you already own it.  You can do all of this by finding the size - in minutes of angle (MOA) - of your sighting system, and learning how to use that knowledge in the field.

To find that size you need a visual reference of known size, posted at a known distance. 



If you download and print the attached file, it will create a sight width measurement tool good at 25 yards, a white wedge inside a dark border. 

If you have iron sights, scan the front post up and down until the front post exactly fills the wedge side to side.  Your front sight width can then be read from the scale.  (If you must use a range other than 25 yards, you can make your own equivalent measuring tool by scaling it up or down appropriately, remembering that 1MOA = 1 inch / 100 yards.  A comuputer graphics program can do this, or you cut out a correctly sized white wedge from regular paper, and glue it onto a a sheet of dark cardboard or construction paper.)

If you have optical sights (or similar electronic sights), you'll need to find the MOA size of a feature on the scope's reticle.  The most common reticle is a duplex:

The thinner lines near the center of the cross hair allow for more precise aiming, but they can also be used for measurement.  You want to size the width of the duplex, usually from the cross-hair to the beginning of the wider line.  You can use the same measurement tool as above, or you may be able to see the grid lines on a standard Appleseed squares target and count off MOA.

The second most common type of reticle is a mildot:

If you have one of these, the work is already done for you:  The center to center distance from dot to dot is 3.6 MOA. 

Now that you know the MOA size of your sights, what can you do with that data? 

Start with adjusting your sights.  Assuming you can see the fall of shot - sometimes a question with iron sights and aging eyes - you can use the sight as a measuring instrument while you are still on the firing line.  As an example, suppose you have a scope where the width of the duplex from center to the wide line is 8 MOA.  You see that the center of your group is right of the point of aim by about half the width of the duplex.  So 4 MOA is a good estimate for the sight correction; on most scopes you would take 16 clicks to the left, and try again.  In addition to saving a walk  downrange, and perhaps a firing period, notice this also saves some math:  You are measuring MOA directly - no need to compute it by measuring in inches and dividing out 100s of yards.

The next two uses of sight size come in long distance shooting.  The first is correcting for wind.  Figuring out windage in detail is another post, so we'll just use Fred's simplest windage rule:  For a significant wind blowing cross range, correct 3MOA at 300 yards, and 5 MOA at 500 yards.  If the wind is fairly constant, you can dial in that that correction on your iron sights or scope.  But in many cases the wind will be gusting, and you'll have trouble making the adjustment in time.  In that case, you will 'hold off' to windward - the proverbial Kentucky windage - using your sights to measure the offset.  If you were at 300 yards, with a cross-range wind, and your front sight post measured 7MOA, holding a little less than half its width into the wind should make the shot.

Finally, you can use your sights to estimate the distance to targets at unknown ranges.  Here's the standard formula for MOA:
  Minutes of angle = (Size in inches) / (distance in 100s of yards).

Apply a little high school algebra and you get:
  Distance in 100s of yards = (Size in inches) / (Minutes of angle)

To figure out the range, you will find the angular size of an object downrange in MOA, and compare it to its actual size in inches.  First you find a reference object of known size, which may be the target you want to strike, or something close by.  If you know your sight's MOA size, you can quickly estimate that object's MOA by comparison. 

Here's where having a mental table of common object sizes is helpful.  How wide is a wooden fence post?  How about a utility pole?  The hub of a car or truck wheel?  How tall is a pop can?  How large is an average adult of the animal you might be hunting?  With this knowledge plus the reference object's MOA, you can get the distance.  Suppose you see an old pickup truck near your target.  Your front sight post is 8 MOA.  You know a truck hub is usually 14-16 inches.  The hub on the truck appears to be about the same width as your sight post.  16" / 8 MOA gives you about 200 yards.  Set your sights or hold over/under accordingly, and strike your target.

Further references on scope reticles:
http://www.gameandfishmag.com/2012/06/15/understanding-rifle-reticles/?pid=700
http://blog.gunspec.com/gear/gunspec-reticle-infographic/


This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License by Tim Oren.
« Last Edit: March 17, 2015, 05:36:16 PM by Nero »