Mathematics For Precision ShootersRecently my old friend Steve Langford of Millett Sights and I discussed the many formulas, measurements, math shortcuts and rules-of-thumb that are of value to precision shooters. Inches M DIN = Inches of change on targetM = Minutes of AngleD = Distance in hundreds of yardsTo use Frank's method, just place your fingertip over the factor you want to determine, which tells you to multiply or divide the other two factors, to learn the third factor. Let's say you want to see how many inches 5 MOAs are at 200 yards: Just place a fingertip over 'IN' and you see that you must multiply the MOAs by the Distance, which means 5 MOAs times 2 (Distance in hundreds of yards). Therefore, the answer is 10 inches.
Thus, changing a sight's elevation setting by 1 minute of angle changes the bullet's impact point by 1.047 inches at 100 yards. And the bullet's impact point would change by 2.094 inches at 200 yards (1.047 x 2) and change by 3.141 inches at 300 yards (1.047 x 3). In this video I try and explain what MOA is or Minutes of Angle in the easiest terms possible. Think of MOA as Inches in Yards if it helps you remember. For a more detailed look at MOA and the.
And if you're trying to determine how many MOAs to crank your scope when you want to raise the bullet impact ten inches at 200 yards: put a fingertip on the 'M' (for MOA), and you see you must divide the inches by the distance. Frank's graphic formula is simple and easy to remember.An Old Marine Corps MethodHere's an old USMC formula for estimating windage compensation, in Minutes of Angle. This only applies to.30-caliber rounds, such as the.308 Winchester and.30-06 Springfield.
2 Moa At 100 Meters Conversion
In use since the days of the excellent 1903 Springfield rifle, this formula is only employed for targets 500 yards or closer, after which the math constant changes. = 2 Minutes of Angle 3 x 10 = 30 15If it's a full-value (90 degree) crosswind, click-in the two Minutes of Angle and aim dead-on; if the wind is oblique, proportionally reduce your clicks. This formula works well except when a target is farther than 500 yards — after that, the Math Constant increases, as shown below:At 600 Yards, Divide by 14At 700 and 800 Yards, Divide by 13At 900 Yards, Divide by 12At 1000 Yards, Divide by 11Compensating for Headwinds and TailwindsIn theory, a headwind slightly increases drag on your bullet and thereby reduces its velocity, while a tailwind has the opposite effect. Thus, a headwind requires that you slightly raise your scope reticle, and a tailwind dictates that you slightly lower it.This formula, cited by W.W. = 7.5 Yards 10 x 3 4Since it's a tailwind, we subtract these 7.5 yards from the 300, and set our sight as if the target is 292.5 yards away - which translates (for a.308 Winchester firing 168-gr. BTHP) to just one click - one tiny 1/4-Minute of Angle click! Had it been a headwind, we would have added that 7.5 yards to the actual distance, and fired as if the target were 307.5 yards away.This may seem like splitting hairs, but headwind/tailwind compensation increases with distance because the bullet slows and its trajectory becomes a plunging arc.
At 900 yards, that same, mild, 10 MPH headwind or tailwind, would require a seven click correction to place our.308 bullet on center-target. And recall that at such extreme range, the bullet is plunging so sharply that even a small elevation error can cause a complete miss.Adjusting for Temperature ChangesThis is another elevation adjustment, dictated by how much temperature variation can change the trajectory of your bullet. Because powder inside your cartridge burns at a higher rate when it's warm, and slower when it's cold, your rounds will strike low in cold weather and high in hot temperatures. The following best applies to.308 and.30 caliber rounds:When the temperature changes 20 degrees from your zero temp, apply 1 MOA at 300 yards;When the temperature changes 15 degrees from your zero temp, apply 1 MOA at 600 Yards;When the temperature changes 10 degrees from your zero temp, apply 1 MOA at 1000 YardsU.S.
Army tests measured the following effects of changing temperatures on.308 ammunition, Degrees FahrenheitMuzzle VelocityBullet Drop at 600 Yards (200-Yard Zero)-102400 fps-109 inches252500 fps-100 inches592600 fps-91 inches1002700 fps-84 inchesA century ago, British firearms authority W.W. Greener developed a formula based upon a 'standard' temperature of 60 degrees Fahrenheit, which is very close to today's 59-degree F. Ballistic table standard. = 15 10 x 3 10Deduct this distance (15 yards) from the 500 yards, then set your sights as if the target is 485 yards away, aim dead-on, and fire.
Altitude EffectsAltitude EffectsSince air is thinner at higher altitudes, there's less drag on a bullet and therefore, in relative terms, a bullet flies faster with a flatter trajectory. Standard ballistic tables assume a sea-level altitude, so already there may be a variance from these tables and the altitude at which you zero your rifle.Concern about altitude change usually involves travel - say you regularly fire your rifle near St. Louis at an altitude of 450 feet, then you go to Colorado to hunt elk at 10,000 feet.
What's the difference? The basic rule-of-thumb is, add or subtract one MOA for every 5000 feet of elevation change.But what about changes in barometric pressure which can vary day-by-day, even hourly, when high- or low-pressure fronts pass through? I don't think there's enough variance to change your elevation except in extreme weather changes; yet, W.W.
Greener, in his 1901 classic, Sharpshooting for Sport and War, suggested, 'One inch of barometric pressure is equal to 15 degrees Fahrenheit change in temperature. Treat a rise in barometer as a rise of the thermometer in the above proportion.' Trajectory Changes for Barrel LengthWhen you consult an ammo manufacturer's rifle ballistic tables, you're actually looking at the velocity when fired through a 24-inch barrel.