Firearm sighting scopes allow the shooter to adjust the sighting of the scope in regular intervals. Each interval, or click, of the scope turret adjusts the scope by a fraction of a measurement called minute of angle. Many scope manufacturers provide wind and elevation adjustment guides listed in MOA. Generally, changing the scope one MOA changes the bullet's impact point by one inch at 100 yards. Using the geometric properties of a circle allows you to convert MOA adjustments to absolute distance accurately to calibrate your sighting scopes.
Determine the exact value of an MOA at 100 yards. Calculate the circumference of a circle with a 100-yard radius by multiplying 100 yards by two by pi (3.14). Multiply the result by 36 to convert yards to inches and divide by the number of minutes in a full circle to determine the exact inches per MOA. A circle contains 360 degrees that consist of 60 minutes each for a total of 21,600 minutes per circle.
Use the range finder to determine total distance to the target. The exact value of a MOA is 1.047 inches at 100 yards. Multiply that value by the total distance to the target. The result shows the exact change in impact point per change in MOA. For example, changing the sight one MOA at 1,000 yards results in a difference of 10.47 inches.
Fire a calibration shot. Aim directly for the center of the target. Measure the total distance from the center point to the impact point on the target in inches. Divide the total distance by the exact value of a MOA adjusted for your shooting range. For example, if you shot a target at 100 feet and you were 3 1/2 inches off then divide 3 1/2 by 1.047. The result, 3.3, shows the number of MOA off target. Record MOA adjustments under common considerations, such as wind speed or elevations, for your own personal reference.
- Photo Credit sniper rifle on the tripod and optical sight image by Vladimir Melnik from Fotolia.com
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