how do you measure POI group size?

[jashobeam]

There has to be more to determining the size of a 5-shot group than simply measuring the distance between the two most widely spaced holes. If, for example, four bullets struck the target in the exact same spot making only one hole and the fifth bullet struck the target one inch away, the group size would not be one inch. Perhaps I am looking to measure the group's average as opposed to its spread? Any terminological assistance would be appreciated also.

From what I've concluded, in order to properly average the distance between points of impact I will need to add all distances between POI and then divide by the number of measurements taken. If using the above example of four shots printing only one hole and the fifth shot impacting one inch distant, I would calculate as follows:

Distance between 1st and 2nd shots: 0"
Distance between 1st and 3rd shots: 0"
Distance between 1st and 4th shots: 0"
Distance between 1st and 5th shots: 1"
Distance between 2nd and 3rd shots: 0"
Distance between 2nd and 4th shots: 0"
Distance between 2nd and 5th shots: 1"
Distance between 3rd and 4th shots: 0"
Distance between 3rd and 5th shots: 1"
Distance between 4th and 5th shots: 1"

Total relative distance: 4"

Divide TRD by number of measurements: 4"/ 10 = .4"

Is this the correct method? What method(s) do others use?

Can some mathematician or statistician help me with my terminology?

 

[M2 Carbine]

I'm sorry but I haven't a clue what you are trying to do.
This five shot group measures 2 1/8 inches.

 

[JoeHatley]

Group size is not normally averaged. It's based on actual measurement.

The group size in your example would be 1".

Joe

 

[azredhawk44]

Jashobeam:

It's just center to center. No fanciness involved. Being off by an inch is exactly that - being off by an inch. You can't average a miss.

You can't average a miss - Seems like that's what Democrats try to do all day long, but still come up with zero.

All hail the truths of the gun culture! Blessed art thou, Sam Colt! Blessed art thou, John Browning! Through your work is true knowledge found!

 

[Jim Watson]

There have been all manner of target evaluation schemes used. One of the oldest is string measure because it was done in frontier days by driving a peg in the center of the target and one in each bullet hole and running a string from center to each shot.
The Army used - may still use - a similar method called mean radius, the average distance each shot is from center.
There are some different figure of merit systems. One, draw a rectangle covering the centers of all hits. Measure the diagonal. Another, measure the horizontal spread and the vertical spread, average.
There are others that have been used over the years, but group size measured center to center of the two widest shots is simple and direct.
If you want to make up your own, feel free.

 

[jashobeam]

Now that you've mentioned it

I think there were some Democrats in the shooting lane next to me. They messed up my shooting and my ability to see the clear and obvious truth. :banghead:

Thanks for the replies. I now know that group sizes are in fact determined by measuring the distance between the two most distant points of impact, and that they have nothing to do with taking into account any of the points in between.

Does no one ever seek to determine the average distance between points of impact? Are all those nameless, faceless little holes destined to be ignored and neglected like ugly step-children?

Also, what constitutes a "flier" or "flyer"?

EDITED: Thanks Jim Watson. You posted while I was typing. I'm glad to hear that someone was aware of different methods. I thought I was on crazy pills.

 

[One of Many]

Group size is indeed based on the maximum dispersion between shots in the group; group count is used as a measure of the value (the greater the number of shots in a given size group, the more value given to the group size).

I think that a better approach to determining the likelyhood of another shot being a certain distance from previous shots would be to use the RSS method. Root of the Sum of the Squares. Take the distance (in thousandths of an inch) between one bullet hole (toward the center of the group) and the others, square those distances, sum the squares, and take the square root of the sum of squares. This minimizes the effect of flyers, and indicates where the likely impact of the next round will occur.

For hunting or defense, you may prefer to know the worst case expectation, which is the flyer that was the greatest distance from the rest of the group.

 

[Gator]

In smallbore shooting you measure the distance from the outside edges of the furthest apart shots and subtract the diameter of the bullet. This gives you the center to center spread.

 

[LHB1]

Quote: "If, for example, four bullets struck the target in the exact same spot making only one hole and the fifth bullet struck the target one inch away,"

Jashobeam,
Description of group size depends on the shooter's veracity. You and I are always honest but I sometimes wonder about "them other guys".

Description of your hypothetical group:
1) You/me: five shot group measuring 1"
2) Them: four shots in one hole and a flyer one inch away
3) Democrats: four shots in one hole. (Nothing mentioned of the fifth shot.) Just a joke guys. I know there are some honest Democrats out there. Aren't there?

Good shooting and be safe.
LB

 

[hksw]

Agree with LHB1. You'll even get some folks saying, "3 rounds, one on top of each other, 0.025" and also a couple fliers."

Also, what constitutes a "flier" or "flyer"?

Somewhat subjective. Shoot 4 rounds into a single hole then touch off the last right when the shooter next to you lets go a round from his .50 BMG. Understandable if there is a flier. However, some (a minority) will do every thing identically from one shot to the next until all rounds are fired and one happens to be off from the others and they'll call that a flier for no other reason than to say it messed up their group.

 

[M2 Carbine]

jashobeam
Also, what constitutes a "flier" or "flyer"?
---------------------------------------------

I like to call this kind of shot a flyer.
That way it sounds like I had nothing to do with it missing.

 

[Aguila Blanca]

If, for example, four bullets struck the target in the exact same spot making only one hole and the fifth bullet struck the target one inch away, the group size would not be one inch.

You are incorrect. In the example you cite, the group size would be 1 inch.

Unless the person describing the group chose to declare a flyer, in which case the group size would be zero inches with a "called flyer" opening it up to 1 inch.

Don't make that which is simple unnecessarily complex.

 

[raytracer]

Gator: I think he's talking about a 5 shot group, not 1 kidney shaped bullet!

Seriously - that's some fine shootin'!

Joe

 

[bluto]

I use the "maximum spread" method like most. It's quick and easy. There are other methods like "Average Group Radius" and "Average Vertical and Horizontal Error" but I've never used them.


Here's a link that describes some other ways to measure your groups:

http://www.shootingsoftware.com/measure.htm

BTW, I've found that fliers are essential for accuracte shooting. All my best 5 shot groups consist of one ragged hole with 4 fliers.

 

[Gator]

Thanks Raytracer! That was from the first time out with the battered old Win 52D I got from the CMP. She is ugly, but she still shoots.