What is the correct way to measure group size? I always measured ourside to outside and subtracted 1 bullet diameter. I have a target that measures 1.325 O to O minus .308 = 1.017, when I measure center to center I get 1.054. This is due to the fact that the .308 bullet doesn't put a .308 hole in the paper. If this were a contest or competition how should the group be scored?

Well since I measure for fun I just try and measure center to center of the outside holes. If you want to measure outside to outside then subtract the diameter of one bullet hole. It will very with the type and weight of target paper. All in the same hole would be 0" not a negative number. Darn wish I needed to worry about that last .037". LOL

You might try over at Benchrest they would know.
http://benchrest.com/wwwboard/

I measure outside to inside on the outside(farthest apart) holes. I mainly do 3 shot groups.

quote:

Originally posted by steve4102:
What is the correct way to measure group size? I always measured ourside to outside and subtracted 1 bullet diameter. I have a target that measures 1.325 O to O minus .308 = 1.017, when I measure center to center I get 1.054. This is due to the fact that the .308 bullet doesn't put a .308 hole in the paper. If this were a contest or competition how should the group be scored?

Steve, What I do is measure one hole of whatever I'm shooting, lets say a 308, see how far out your calipers are on the hole to make it a .308" hole, then when you get a group going you know where to place your calipers on or should I say on the outside of the group to get an accurate measure. It's usually about 1/32 off the carbon mark left by the bullet as it passes through the paper. Jay

What I do is take the two farthest holes and measure one sid to the corresponding side of the other hole (ie put a ruler in the middle of the two holes and measure the left sides).

If I'm having a good day and all the holes touch then I'll shoot a round into the target near the group but not touching it, then measure that hole. Measure the group, then subtract the diameter of the lone hole.
If you subtract the diameter of a bullet then you'll get a measurement that is smaller than it really is because the holes are rarely the diameter of the bullet.

If you were shooting in a registered Benchrest Group match, the groups would be measured outside to outside, at the widest point, and 1 bullet diameter would be subtracted. The person measuring the groups is just one more person that you are competing with. If the scorer adds .010" to your group and subtracts .010" from mine, you are instantly .020" ahead of me in the agg.

GROUP SIZE ANALYSIS METHODS

(Maximum Spread)
The Maximum Spread (also called group diameter or extreme spread) is simply the distance between the centers of the two most widely dispersed shots in a group. This is the most common method used to describe group size because it is the easiest measurement to take. Unfortunately it only considers two shots in the result so an otherwise tight group of 10 shots with one "flyer" can produce results similar to a widely dispersed two shot group. For bench rest shooters who are capable of putting five shots in one hole it is a valid measure; but for the rest of us, or for handgun shooting it does not provide a "statistically significant" number for comparing targets.


(String Measurement)
This is an old method still used to determine a shooter's skill at hitting a target. It assumes the point of aim is always the desired point of impact and is simply the sum of the distances from the point of aim to each bullet hole. Originally a string was used to gather the distances, hence the name. It is still a valid measure of total error relative to the aim point. String Measurements however cannot be used to analyze sight settings because it only measures the magnitude of error, not the direction of error. It is also not a useful measure of group size because a tight group located away from the Bullseye will produce a large String Measurement.


(Average Group Radius)
The average of the distances from the statistical group center (not the aim point) to each shot is the Average Group Radius. It accurately reflects how far a typical shot will impact from the aim point with a well sighted arm. This measure is the best indicator of group size or firearm performance because flyers have less impact on the result so it provides a "statistically significant" number for comparing a series of targets or groups. Unfortunately it is difficult to calculate manually. Unlike a String Measurement, the Average Group Radius does not assume the point of aim is the desired impact point, so sighting error is not a factor in the result. Average Group Radius can therefore be used to compare group sizes when the impact point is intentionally high (rifle sighted for maximum point blank range and shooting dead center). It is also the best measure for evaluating the overall accuracy of loads. Magazine articles using Average Group Radius indicate the writer has taken care to provide readers with statistically significant data, but you should be careful not to compare it to the more common Maximum Spread.


ERROR ANALYSIS METHODS

(Vertical and Horizontal Spread)

The Horizontal and Vertical Spread of a group is simply the greatest distance between shots on the vertical or horizontal plane. This contrasts with the Maximum Spread which can be at any angle across the group. This measurement is understood by most shooters, easy to make, and can be used to help detect load and mechanical problems or "pulling" by the shooter. If the Vertical or Horizontal Spread is significantly larger and shots are well dispersed, it is called "stringing". A cross wind will obviously disperse shots horizontally. Vertical stringing may be caused by irregular powder charges and detonation problems (inconsistent ignition due to variations in primer pocket depth or primer thickness). Improper crimping, bullet inconsistencies or other loading problems usually result in larger overall group sizes, and not "stringing" in any particular direction. A loose gun sight, or broken scope may also cause stringing.


(Average Vertical and Horizontal Error)

The Average Horizontal and Vertical Error is the average of errors on the Vertical and Horizontal plane from the group's statistical center. This is a much better measure for detecting errors than Vert./Hor. Spread because the shot data is averaged to reduce the influence of "flyers". If the goal is to track "pulling" by the shooter over time or to isolate problems, then the Average Vertical & Horizontal Error is a better measurement for comparing a series of targets.


(Maximum Shot Radius)

The Maximum Shot Radius (or maximum group radius) is the distance from a group's statistical center to the center of the most distant hole. It really only indicates how far from the group center the worst shot should fall and is not a good indicator of overall performance. On a target where shots are evenly dispersed, this measurement will be about 1/2 the Maximum Spread and larger than the Average Group Radius. It is best used to quantify the worst shot in a series of targets.


(Maximum Shot Radius Compared To Average Group Radius)
The Maximum Shot Radius should be only slightly larger than the Average Group Radius. A big difference between these measures is another indication of shooter error or a bad load.


(Average Elevation and Windage Error)
The Average Elevation and Windage Errors compare a group's statistical center to the point of aim. It accurately indicates where the group center is located (or average shot impacted) with a set of vertical and horizontal dimensions. The two measurements show how well the firearm is sighted to hit the Bullseye and can be used to adjust sights if the results are converted to Minutes of Angle.