Charlie Foxtrot

This can can a bit scientific but here goes. First the concept of what “Minute Of Angle” MOA is and how it affects the bullet’s dispersion pattern as ranges are increased must be understood. I found this on another website and I believe it is close and can be helpful.

True Minute Of Angle, MOA, is measured by first determining the minutes of angle in a circle. We know that a circle contains 360 degrees and that there are 60 minutes in each degree. The minutes of angle in a circle is 360 x 60 or 21,600. The circumference of a circle is 2 x p x R where R is the radius of the circle in inches. Hence, for any range R, a minute of angle equals 2 x 3.1416 x R divided by 21,600 or .000291 x R.

Therefore, if the range R is 100 yards, a minute of angle would be .000291 x 100 x 36 or 1.0476 inches. Because the distance covered at 100 yards is so close to being exactly 1 inch, the fraction is usually disregarded for range work, and we commonly say that a minute of change on the rear sight will move the point of impact 1-inch at 100 yards. If we were shooting at 1,000 yards, a minute of angle would cover .000291 x 1000 x 36 or 10.476 inches. The difference between the true minutes of angle and the rounded minutes of angle becomes greater as the range increases.

So basicly 1 MOA equals = 1.045 inches at 100 yards, and increases as the range increases. Most round it to 1 inch – 1 MOA just to ease the process. And except at the “benchrest or at extremely long ranges” the 1″ works fine. At least for me anyway. Examples: 200 yards = 2 MOA, 300 yards = 3 MOA… so forth and so on. 500 yards is 5 minute of angle MOA.

To find the theoretical increase in elevation at 100 yards to hit line of sight – point of impact from 200 to 500 yards, divide the range by “100″ and you get the MOA for that range. 500 yds is 500/100=5 MOA, 450/100= 4.5 MOA, 475/100=4.75 MOA etc etc. Just think of MOA as one inch, it makes it easier.

If your bullet drop at 500 yards is 62″, then divide 62″ by 5 MOA and you get 12.4″ or 62″/5MOA=12.4″. Given this ballistics input(AKA: Dope), if you zero your sight at 100 yards to impact 12.4″ high, you theorectically should hit point of impact – line of sight at that range.

That is what the reticle takes into account in the scope. You wrote,

At the bottom it shows the reticle with standard corrections at 100 yards of 1.5″ at 200, 4.5″ at 300, 7.5″ at 400, and 11″ at 500 yards.

I compute similar data below manually.

Range – Bullet Drop – MOA
100 – zero point of

impact – line of sight
200 – -3.3″ 1.65″ below line of sight
300 – -13″ 4.3″ below lin

e of sight
In order to correct for the bullet drop at the farther ranges, simply zero your weapon at 100 yards, then crank the scope up the appropriate amount of MOA’s or inches from above chart, and you can reasonably be expected to hit at the given farther ranges.400 – -32″ 8″ below line of sight
500 – -62″ 12.4″ below line of sight

I don’t like using holdover, mildots or stadias, I much prefer what we call, “cranking in the ballistics dope” to achieve a “line of sight – point of impact” hit. That way, my reticle cross hair never leaves the desired point of impact.

You can also use the same method for determining the exact hold off for wind drift. Stadias and mil-dots can also be set-up to provide for that as well. But that is another thread.

The Bullet Drop Compensator BDC, does the same thing as above. It is pre-adjusted, geared I believe, and measured for the ballistic characteristics of a given round. All you need to do is estimate the range… crank to that range on the elevation turret, hold center and squeeze. Given a no wind situation of course.

Finally, what ever ballistics dope you use, or method of acquiring and then applying that dope for the given load… you must then go to that “given” range, to confirm that it is actually right for your set-up. Then adjust, record for later use, and then practice, practice!

Know this: temperature, humidity, altitude, and even the distance to target all serve to change this dope in day to day varying conditions. I suggest that you keep a solid accurate record of your results each and every range visit, recording those variables. Then you will have built a solid mechanism for returning to the “line of sight – point of impact” desired under varying conditions and ranges.

I know this was wordy, and hope it is helpful.

Others probably have a better way of explaining this than I.

A mild, erratic breeze of 5 miles per hour can open up 100-yard groups over half an inch — in addition to whatever accuracy your rifle, load, scope and shaky self are capable of producing.  And that’s when shooting a high-velocity rifle with a relatively heavy bullet with a high ballistic coefficient.  A .223 Remington with a relatively low-BC varmint bullet, or any rimfire cartridge, is much more susceptible to the vagaries of the wind.

Anything helps, even a strip of flagging tape hung from your chronograph, but when things get serious I haul out some Sinclair flags that combine a sheet-metal wind vane, a plastic “sunflower” that spins off the front of the vane, and a strip of flagging tape off the rear of the vane.  These will turn in the slightest breeze, and the flagging tape and spinning sunflower provide a very good idea of relative wind speed.

There’s some minor controversy about where to set wind flags, but the consensus is that winds closer to the muzzle make the most difference, since they start working on the bullet sooner.  This is why flagging tape hanging from the chronograph can help a lot.  When using two of the Sinclair flags, I put one 10-15 yards in front of the bench, and the other about 60-70 yards out.

One technique is to watch the flags for a few minutes to find the prevailing wind “condition.”  After that you only shoot during that condition.  Even factory rifles will shoot some astounding groups when the shooter is aided by wind flags.  Among the “plinker” rifles in our house is a Bushmaster ORC in 5.56mm / .223 Rem. with a 16″ barrel and an A2 Bird Cage Suppressor.

The most basic measurement that a flag is good for is determining actual wind direction. This essential measurement will help you to determine what value to give to the wind; full, three quarters, half, or no value. Wind direction is determined relative to the shooter’s position using the clock face method, or using the angle measured in degrees. When the wind is blowing at 90 degrees (3 o’clock) or 270 degrees (9 o’clock) relative to your shooting position, we assign it a full value of 1. Wind blowing at 45 degrees, 135 degrees, 225 degrees, or 315 degrees relative to your position is given three quarters value. When the wind is blowing at 0 degrees or 180 degrees (12 o’clock or 6 o’clock) relative to your position it is disregarded and given no value.

Once wind direction and value is determined, it’s time to measure or estimate the wind speed. An anemometer is probably the most accurate device for measuring wind speed, but there are other methods that you can learn. If you find yourself without an anemometer, you can use the guidelines set forth in the Service Rifle Pamphlet produced in 1931 by the US Army Infantry Team. While the information is old, the guideline is as valid today as it was 79 years ago.

0-3 mph   Wind hardly felt, but smoke drifts
3-5 mph   Wind felt lightly on the face
5-8 mph   Leaves are kept in constant movement
8-12 mph   Raises dust and loose paper
12-15 mph   Causes small trees to sway

Flags can also be used as a rough estimate of wind speed. When observing a normal rectangular flag, estimate the angle between the flag and the pole and divide that number by 4 to get the approximate wind speed. For example, if a flag is flying straight out at a 90 degree angle, the approximate wind speed is 22.5 mph or greater (90/4). If the flag is limp and flapping in a breeze at a 45 degree angle to the pole, the approximate wind speed is 11 to 12 mph. This same estimation method can also be used for streamers and pennants.

As important as knowing how to read the wind is knowing your cartridge and how your load will be affected by various wind speeds. Many novice shooters simply do not understand, or do not believe, how much of an effect a cross wind can have on even the speediest of bullets. Consider a 55 grain .223 round fired down range at over 3,250 FPS for example. With only a modest 5mph cross wind that little .223 bullet will be pushed over 1/2? off target at only 100 yards. While that might not seem like much, consider that a 10mph wind will result in the same round being pushed more than 1 MOA at any range. Experienced shooters, having been frustrated by wind before, often have the opposite problem and tend to overestimate the effect wind will have on their bullet.

All bullets have a ballistic coefficient that is usually computed by the manufacturer. This number, combined with the flight time of the bullet, can help you determine how much your bullet will be affected by a given wind. By combining the wind direction and value, speed, flight time and the ballistic coefficient of your bullet, you can determine how much to hold over or how much to adjust the windage on your sights. Because of the fact that bullets with differing ballistic coefficients are affected to differing degrees by the wind, there is no hard and fast rule for calculating wind drift. I won’t get into the mathematics of computing wind drift using the ballistic coefficient and flight time of your bullet; wind drift charts and calculators are readily available for almost every cartridge load. Use a wind drift chart for your specific load to determine how much holdover or windage adjustment is necessary.

With the information from the appropriate wind drift chart, apply the wind value to determine the actual drift. For example: Say our chart shows that M2 match ammunition for an M1 Garand from American Eagle will drift approximately 5.8 inches at 600 yards with a full value wind at 1 mph. If we actually have a 10 mph wind blowing in at a 45 degree angle (1:30 o’clock) we assign it a value of 3/4 and do the math (5.8 inches X 10 mph X .75) to arrive at 43.5 inches of drift. If the wind shifts to be 30 degrees (1 o’clock) we would assign it a value of 1/2, resulting in 29 inches of drift. Doing the math, we correct approximately 5 MOA for wind at 1/2 value and 6.9 MOA for 3/4 value.

http://www.ballisticards.com/