The AR-15 series is a Rifleman’s rifle. It is accurate, customizable, and easily maintained. Even with off-the-shelf Full Metal Jacket (FMJ) ammunition, it’s a 3-MOA gun. In other words, a 5.56mm can hit a man-sized target at 600 meters without any customization or match ammunition. As many users have verified, upgraded stocks, optics, and ammunition can extend this range. This accuracy holds during good weather.
During wind, the burden of accuracy shifts from the AR-15 (accurate 3MOA or better), to the shooter or spotter who is calling the wind. Out to 100 meters, the wind would need to blow at roughly 60mph to move a bullet off of a man-sized target. Even at 300 meters, the shooter can get away with holding on the left edge or the right edge to compensate for any wind under 15mph.
At 400 meters and further, incorrect wind calls will lead to misses. A mere left or right edge hold limits the shooter to a 7.5mph wind. In other words, the AR-15 shooter’s lack of wind calling skill limits the platform’s effective range to 400 meters on a significant number of days.
The Rule of 7 for 5.56
The Rule of 7 for 5.56 is simple. To get your windage hold, multiply the meter-line and the windspeed in mph, and divide by constant 7.
500 meters, 4mph wind, full value. Need MOA correction.
MOA = 5 * 4 / 7 = 2.85. Round to 2.75 or 3.0 MOA
Now, you need to know the value of the wind. Your gun is pointed at the 12 o’clock. 12 and 6 are no value. 3 and 9 are 100% value. 11, 1, 5, 7 are 50% value. 2, 4, 8, 10 are 70% value.
620 meters, 8mph wind out of the 1’oclock. Need MOA correction.
MOA = 6 * 8 * 0.5 / 7 = 3.43. Round to 3.5 MOA
Perhaps you don’t have a scope or reticle that works in MOA. You want inches so that you can hold off of the target into space. Remember, shoulder-to-shoulder is 19″ on an average male. Simply multiple the MOA correction with the meter-line.
400 meters, 16mph wind out of the 4’oclock. Need inches.
MOA = 4 * 16 * 0.7 / 7 = 6.4. Round to 6.5
Inches = 6.5 * 4 = 26 inches of hold.
Accuracy Check with Sendit Ballistics
We’ll use Sendit Ballistics, my iOS app, to check the accuracy of the rule, assuming 62-grain 5.56 at 2970fps. Wind is full value 10mph.
200 meters: 3 MOA by Rule of 7, 2 MOA by Sendit Ballistics
400 meters: 6 MOA by Rule of 7, 4.5 MOA by Sendit Ballistics
600 meters: 8.5 MOA by Rule of 7, 8.0 MOA by Sendit Ballistics
800 meters: 11 MOA by Rule of 7, 12.0 MOA by Sendit Ballistics
In other words, keep the Rule of 7 in your back pocket for windy days! And keep Sendit Ballistics in your back pocket too, in case you need perfection.