How does one measure group size?

[Zak Smith]

I'm pretty sure a CAD system, graduated rule gauges, is not needed to practically measure group size.

Anybody who understands how to use a ruler can find the pair of holes with the maximum distance. And I can just as easily be done for any number of shots, not just 4.

 

[Zak Smith]

Otherwise, does anybody know a simple algorithm, by which I can compute the actual group size of 3 shots on a spread sheet?

Superimpose a grid on the shot target. Define 0,0 whereever you want. Get the position for each shot fired (center) by whatever method you want: P1=(x1, y1), P2=(x2, y2), etc.

The distance between set 1 and set 2 is:
dist(P1,P2) = sqrt( (x2-x1)^2 + (y2-y1)^2 )

then the group size would be MAX( dist(P1,P2), ... , dist(Pn,Pm) )
where every combination of points is represented.

By similar methods, you could relate every sho to the POA, or the geometrical center, etc.

But the important thing is the group size, which you only need a ruler/caliper and maybe a calculator to figure.

-z

 

[cropcirclewalker]

Sorry, Mr. Smith, but not so, not so.

If Mr. OneInchGroup is correct, which I have no reason to doubt, 3 bullet holes equally spaced around the origin at a polar distance of 1 inch would have a group size of 2 inches. If, however, you measured the distance between them, it would measure up at 1.7321 inches.

Yes, I can do the pythagorical solution dist(P1,P2) = sqrt( (x2-x1)^2 + (y2-y1)^2 )


The question remains........How to draw an arc through the center of the most remote 3 holes, mathimatically.

Like I said, 4 is easy. The 2 most distant (with the other 2 inscribed) is the group.

 

[Zak Smith]

Ah, I see what he's trying to get at now. Thanks for the explanation.

I've never seen it defined as the size circle that can cover all the impact centers before.

In fact, the NBRSA doesn't either,

http://64.233.167.104/search?q=cach...f+national+benchrest+scoring+group+size&hl=en

GROUP MEASUREMENTS. Groups are to be measured by any method approved by the
NBRSA in .001 inch. The Sweany Type Reticle Rule (or its equivalent)
will be the only official measuring device used at all Registered
Matches. In measuring groups fired with calibers larger than .22
(unless the Reticle is calibrated for the caliber to be measured) the
measurement shall be made from the extreme outside edges of the 2
widest bullet holes and the actual differential of the larger calibers
shall be subtracted from the measurement read on the measuring scale.

-z

 

[OneInchGroup]

How To Measure Group Sizes

Well, Zak, looks like you've put your finger on an area of disagreement amongst the various target shooting Associations. Over at ASSA, the plan was to try to match up with the type of judging done in Bullseye competition, where the scoring is based upon shots being inside one or another of the numbered rings, smallest being best score, etc.

The NBRSA method simply measures the center distance between the two shots in a group that were farthest apart, a one-axis Caliper measurement that adjusts for the size of the bullet. The advantage is that the measurement is so simple that it can be done in the field without any complicated calculations required. It also gives an artificially good result compared to running a circle gauge.

Depending upon what you are used to doing, either approach is fine, as long as everyone understands that you are comparing apples and oranges.

Here is why the two methods can give very different scores:



Just to make everybody happy, ASSA has decided to include results calculated both ways, so everyone can know where they stand, no matter whose method is used. We think the NBRSA method gives "Rosy Scenario" results in some situations, per the example we've posted, but they've been at it a while, and we'll just make another score column to try to be all-inclusive, "big tent" kinda guys.

 

[cropcirclewalker]

To correct my previous post. A 4 shot group which has 3 of the holes as shown above and with the additional 4th shot somewhere included in the circle would have the same effect as if the 4th shot was not made.

If the shots were "stringed" like mostly in a line, none of this would matter even to the extent that a circle with its diameter drawn through the center of the most remote 2 holes would qualify with both methods of measurement provided all the other holes fell inside it.

So, there we have it.

The only time the two methods are in conflict is when there are 3 holes which when an arc is drawn through their centers would create a circle which is larger than that which would be created by drawing a circle through the 2 holes that are the most distant.

We could be talking about zillionths occurances here, but I can see where it could occur.

Fortuantely, most of my groups are sort of stringed. So the conflict may be like the fly crap in the pepper.

Yet, if necessary, I could plot out the holes on my cad system and let it work its wonders.

I guess I will stick with Pythagoras.

Thanks both of youse guys for the help and clarification.

.

 

[Zak Smith]

The fundamental difference between Bullseye or High Power vs. Benchrest is that Benchrest does not care exactly where the bullets go, as long as they are in the correct target region, while Bullseye & HP shooters shoot at a specific aiming point (the X) surrounded by scoring rings. Bullseye and HP are not scored based on group size, they are scored based on, well, score on the defined target. Different games with different goals.

 

[hipwr223]

In highpower it is not always clear either. To score a higher value in NRA highpower the shot must be tangent to the line of the next higher scoring ring. I have seen so many shots so close that even with a plug it is hard to make an accurate measurement. The nice thing about highpower VS bench rest for scoring is that if the shot is SO close that it requires a plug to tell if the shot is tangent to the line then "most" shooters will simply give the benefit of the doubt to the shooter. Trying to measure groups to some infinitely small measurement seems to be futile to me, but then again I am not interested in BR.

 

[Zak Smith]

Yeah, there is that call if it broke the next scoring ring or not.

But my point was that HP and B. neither score based on group size as such.

 

[Jon Coppenbarger]

Just a different game thats all. I even watched a 10 meter air rifle and pistol match at the OTC on sunday. They had scoring rings also. Well the 10 ring was just a dot in the middle on the rifle target. I loved the shoot offs when you score in 10ths. like a 10.5 or 10.8 or what ever. Interesting watching all those great shooters from up close. how many times do you get to watch shooters from Russia or the Czech team shooters or our best from 20 feet away.
More olympic medal and world record holders and any other type of champion you could think of shooting. Humbling experience in the least. If they use scoring rings and count the rings instead of measuring groups I guess it is good enough for a duffer like myself.

John I got my space gun back and putting new sights on it this week and I plan on getting used to those apeture sights. Should be fun.

Jon

 

[WayneConrad]

Group size is a useful simplification, because no math is required. That makes it the universal currency of measuring consistency.

But if you're going to use a computer, you can invent other measurements. For example, you could determine the geometric center of the group, compute the distance from it to each shot, and then compute the mean, median, and max of those distances. You could do a barchart of the distances.

What would that tell you? Heck, I dunno. But it'd be fun to try.

Somewhere, someone's got a program that lets you scan your target, puts the image on the screen, and then click on the bullet holes on the screen to tell it where each shot landed (a poor man's digitizer). Then it does calculations for you.

 

[milanuk]

Wayne,

Sounds like the Target Analysis module in RSI Shooting Lab... you go thru a 'calibration' step to take into account some specific characteristics of your monitor, and then basically you put the target up against the monitor, and then click the mouse cursor under each bullet hole. Obviously some room for error there; don't really think it's meant for measuring say, point-blank BR targets, but it works fairly well for some other things, like ladder load tests where you are looking for vertical 'plateaus' as you can isolate the horizontal and vertical x/y coordinates of the center of the group vs. the point of aim. I've used it to analyze some targets that would have been too close for me to call by eye, and it's helped me find that 'sweet spot' pretty much dead on. It has a couple different methods for calculating group size, including mean radius, string method, etc. Whatever floats your boat. Fun to play with, anyway.

Monte

 

[bdutton]

I measure outside edge to inside edge of the two furthest holes. No subtraction necessary.

 

[OneInchGroup]

How To Measure Group Size

bdutton,

Works fine if everyone is using the same caliber bullets and not working to the "ring measure" method. In our contests we have to allow for anything from the .44 cal sabot to a .410 gage solid slug to a 12 gauge solid, etc., so measuring the outside would always be an unfair measure to the guy firing the biggest bore.