Calculating Rifle Precision
First, precision and accuracy are not the same. A rifle grouping 1/4 MOA is very precise for a firearm. If that 1/4 MOA rifle puts groups off the aiming point it is not accurate. Rifle precision is a measure of how close it will hit to the same point it had before. Rifle accuracy is hitting the aiming point. Equally though, it is true that if you are shooting for groups and do not wish to destroy the aiming point but rather have the shots hit somewhere else on the target the rifle is accurate if it does what you wish. All of the above assumes, of course, that no blame can be proven on the operator of said rifle.
For the purpose of determining rifle precision I will discuss only two methods. The 'extreme' or 'maximum spread' method is the usual and quickest method used by most of us while in the field. It is not accurate in fairly determining the performance of a rifle but becomes a less inaccurate method as group size decreases. For very tight groups it is a good method of measure because the distinction between it and the 'average group radius' diminishes. The 'average group radius' method is the best and most difficult I know of in determining rifle precision. Determining average group radius involves time consuming measurements and calculations in order to determine center of group and then average shot placement relative to that center. It is a highly accurate means of calculating rifle precision.
77 Yard Target
calculations are compliments of RSI Shooting Lab Software
Using the Maximum Spread Method the above 3 shot grouping measures 0.70 MOA; the Average Group Radius is 0.33 MOA.
Average Group Radius tells us how far from center of group we can expect the shot to be. Maximum Spread emphasizes the worse shot.
Sometimes referred to as 'extreme spread' maximum spread is a measure of the distance between the centers of the two furthest shots within a group. It is often thought to be a measure of the smallest circle the group will fit into. This is misleading because the method does not distinguish between a tight group that has a dissimilar shot and a not so tight group that is evenly dispersed. The maximum spread method is acceptable for measuring tight groups that do not have so called flyers and a reasonable measure for groups with all its shots evenly dispersed. While in the field, I will usually rely on this method for a quick idea of what the group is. If the group is interesting enough or important enough I will later take the time to calculate the average group radius.
Average Group Radius
First find the group center. Measure distance from center of group to center of each shot. Average the distances. The result is the average group radius. Double that would be the average diameter of the group but this figure is not normally expressed. The average group radius is the average distance of the center of shot to center of group.
To manually find the center of group draw a horizontal line through the center of the lowest shot. Draw a vertical line through the center of the leftmost shot. Measure and add the distances from the center of each shot to the horizontal line. Average the sum of these distances by dividing it by the number of shots. This gives the distance from the horizontal line to the vertical center. Measure this distance from the horizontal line and mark that location on the target. Measure and add the distances from the center of each shot to the vertical line. Average the sum of these distances by dividing it by the number of shots. This gives the distance from the vertical line to the horizontal center. Place rule across vertical line and mark made at vertical center. Measure the distance of the horizontal center from the vertical line and mark it on the target. This last mark is the statistical center of the group.
There is a ballistics program available that includes the unique feature of entering a target grouping into the program. It will analyse the group and give statistical data on it. Included in the analysis is the average group radius. It is easy to enter. You press the target to the video screen and click on each bullet hole with the mouse. You can also scan the target in. There is a feature the software includes to calibrate the aspect ratio of the video input so that the distances in both horizontal and vertical directions are correct. The software can be found at Recreational Software Inc. and is the software used to analyse the above target.
A further discussion on measuring rifle precision is provided by selecting 'Measuring Targets Tech. Article' at Tech. Articles.
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Other Topics available:
Wood Stains and Gun Stocks
Floating the Barrel
Barrel Bedding Block
Inlays, Tips, and Caps
What is M.O.A.?
Calculating Rifle Precision
Target Crowning a Muzzle
Building a Muzzle Loader from Kit
Eliminate Trigger Over-Travel
Attaching the Recoil Pad
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