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Posts Tagged ‘rifleman’

Rifleman basics: sight width and battle sight zero

22 January 2011 9 comments

What is the width of your rifle’s front sight, in minutes of angle? How can you use this bit of information to find a target’s range?

First, a minute of angle (hereafter, MOA) is 1/60th of 1 degree of a circle, or 1/21600th of a circle. You can calculate your sight’s MOA width using this formula*:

(21600 X W)/(2 X pi X R) = MOA

where W is the sight width and R is the sight radius, both in inches. To calculate range, use this formula:

(S X 100)/MOA = R

where S is the target size and MOA you calculated from above.

Why is this useful? Because in order to hit your target you need to know whether it is within your default range, also called your Battle Sight Zero or BSZ. Bullets travel in arcs, not lines, and the Battle Sight Zero is the maximum range at a given sight setting which a center hold sight picture will produce a hit – anything closer than that will be shot, anything beyond that range will not be. To engage targets beyond your BSZ, you need to either adjust your sights or hold over your target a given amount. Naturally, adjusting the sights is more precise.

The BSZ extends if you raise your rear sight and drops if you lower it, so you usually keep the sights set at a given range. If you can calculate the range to target quickly, you can determine if you need to adjust your sights, then take the shot, confident that you will hit the target. On the M14, the BSZ is designed to be 300 yards (275 meters) with 147 grain ball ammo.

Let’s take a real-world example. The front sight width for the nearest AR at hand is 0.067″, and it has a 20″ sight radius. This gives a sight width of 11.5 MOA. Suppose I’m shooting at a 20″ steel target at unknown range. I can tell from my sight picture that the target is half the width of the sight, so about 345-350 yards, which is about the maximum effective range of the rifle. Suppose I wait until the target is the full width of the front sight. This puts the range at about 175 yards, half the distance. The problem scales linearly.

Notice that I say “about” a lot. I’m rounding to take into effect the error in target size estimation. The exact numbers are more precise than most people can accurately determine and shoot, including myself.

The wonderful thing about the AR platform is that there are a zillion different parts that are easy for anyone to swap out, including the front sight. My job now is to find a sight with a width such that a 20 inch target at 250 yards appears the same width as the front sight.

Which would mean the front sight needs to be how wide? Anyone? Anyone?

Bueller?

*Both formulae are taken from Boston’s Gun Bible, chapter 18. If you don’t have this great book, or if you have an old one, click the link and get yourself an updated copy.

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Rifleman exercise: target detection

20 January 2011 9 comments

This link will take you to a picture displaying a set of targets at a range of 300 yards. The targets are 20″ steel plates. Time yourself from when the picture loads to when you pick out the targets.

Of course, there is a handicap to take into account, since you’re viewing a picture on a computer with varying resolutions, image sizes, etc. etc.

But the next time it snows in your AO (or tomorrow, if there’s still white stuff on the ground), go out and put a coat on such a target at such a distance. Better yet, have someone else do it for you and repeat the exercise in real life.

Now answer this: what sight adjustment would you have to make to hit your target, when adjusting from your rifle’s normal sight setting?

What do you mean, “I don’t know!”?

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