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Old May 24, 2009, 10:02 AM   #1
bulldog1
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What and how to measure a shot group

I have been shooting just over a year. I see publicatios that show a one inch shot group in a couple of ways. They are scattered or in a cluster. Sometimes inside a 3in radius or 5in radius target. I put 20 0f 25 shots in a 3in radius. So am I shooting a 3 in group? If not, what are the standards to gauge ones shot group.
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Old May 24, 2009, 11:12 AM   #2
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I was taught to measure the maximum width of the grouping and subtract one bullet diameter. So if your group maximum spread is 1.5" with a 30 caliber bullet then 1.5 - 0.308 = 1.19" group
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Old May 24, 2009, 11:19 AM   #3
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Sounds about right olmoss

The other way is to measure from the left side of the left most hole & the left side of the rightmost hole.... then from the top of the topmost hole, to the top of the bottom most hole.... just read the measurements

Saves having to subtract the caliber (especially if you flunked kindrgarten like I did)
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Old May 24, 2009, 12:08 PM   #4
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I'm with Dingoboy!
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Old May 24, 2009, 04:13 PM   #5
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So, do you take the smallest of the two, the largest of the two or do you average the two?
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Old May 24, 2009, 04:26 PM   #6
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Typical group measurement is center-to-center of the two holes farthest apart in the group.

That's if someone is talking about group sizes. There are other ways to measure accuracy, but usually there will be some pretty obvious hints that they're not simple group size measurements.
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Old May 24, 2009, 04:47 PM   #7
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Quote:
Typical group measurement is center-to-center of the two holes farthest apart in the group.
That is the same as my measurement without the arithmatic. I find it easier to align the hole edges than to estimate the centers. Obviously, my groups are somewhat larger than yours.
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Old May 24, 2009, 06:28 PM   #8
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Thanks for the replies gents.
So i need to take about 5 shots and measure the diameter of them as Dingoboyx suggest.
I just don't like math.

Keep 'em down range!!!
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Old May 24, 2009, 06:49 PM   #9
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Going back to your original example, "20 0f 25 shots in a 3in radius", it's reasonable, especially when the number of shots in a group is quite large, to throw out a few shots if you "called" them as being bad shots.

Calling a shot bad means that when the shot broke you said to yourself: "That shot was high and left." It doesn't mean you shoot a bunch of shots, go down to look at the group and say to yourself: "If I throw these two shots out my group size goes down by half."
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Old May 24, 2009, 09:06 PM   #10
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Im sure Im wrong here, but I think of a circle (with the first shot being the centerpoint) and all following shots as "endpoints" which then connect back to the first "centerpoint" shot (ie., forming a radius). From there, I calculate a basic shot group in inches.

I tend to take the best 3 shots (within a group) when considering the potential accuracy of a weapon.
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Old May 24, 2009, 10:19 PM   #11
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25 shots in a 3" group

there wouldnt be much/any paper left in the 3" circle

Now thats what I like to see
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Old May 25, 2009, 07:36 AM   #12
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Quote:
JohnKSa:

Going back to your original example, "20 0f 25 shots in a 3in radius", it's reasonable, especially when the number of shots in a group is quite large, to throw out a few shots if you "called" them as being bad shots.

Calling a shot bad means that when the shot broke you said to yourself: "That shot was high and left." It doesn't mean you shoot a bunch of shots, go down to look at the group and say to yourself: "If I throw these two shots out my group size goes down by half."
You mean "flyers!"

Do I get a star?

Yes, center to center, up down, right to left. Get out the old Texas* (R).
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Old May 25, 2009, 10:05 AM   #13
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Note how most people are giving you diameters (measuring the two holes farthest apart). Perhaps those stating a radius are just mis-speaking. But a 3 inch radius is a 6 inch diameter for the same circle.

So I propose always dividing your diameters by two and reporting radiuses! To those who hear "diameter" in their mind you'll seem twice as accurate.
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Old May 25, 2009, 04:52 PM   #14
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First I measure the distance shot and multipy by two, then I measure the two holes furthest apart, then divide by three, and then lie about how good a shot I am on the internet.
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Old May 26, 2009, 08:06 AM   #15
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There are a number of problems with the traditional way of evaluating firearm accuracy (it's really precision - accuracy is achieved by adjusting the sights) using the "group size" method. (I recognize this thread is in the handgun section, but group size analysis is really something that is done more seriously with rifles, even though the same procedures work for either.)

The first is that, no matter how large your group, you're essentially only using the information provided by the two shots that fall farthest apart. There's much more information available if you can somehow factor in what the other shots are telling you.

Second, the group size is unavoidably linked to the number of shots you fire - on average, 3-shot groups from the same gun must be smaller than 5-shot groups, which must be smaller than 10-shot groups, etc. Related to that is the fact that the group can never get smaller as you add shots to it, only larger. There's something inherently unsatisfying about that.

Third, and perhaps of lesser concern, you can only compare groups of the same number of shots to each other. If you shoot a 5-shot 1-MOA group with one gun and a 3-shot 1-MOA group with another, which is more accurate? What would you expect either gun to do if you shot a 10-shot group?

Finally, how do you really use the information provided by the group size method for determining practical accuracy in the field? For example, if you have a gun that averages .75-MOA 5-shot groups, what's the statistical probability that a shot taken at 300 yards will strike a target 6 inches in diameter? There's no way of answering that question unless you have more information - and that information is available to you if you want to use it.

Oh - one more. How do you use the information from your group size to adjust your sights so the gun is hitting where you want it to at the distance you're shooting at - not by guess and by gosh, but properly and rationally?

There is a better way, but like most better ways it involves a bit more work. Here's how I evaluate my groups:

Fire a group of any size at an aiming point, then use a ruler (I find digital calipers are actually easier) to measure the distance (i.e., the distance from the aiming point to the center of the shot hole) horizontally (on the x-axis) and vertically (on the y-axis) of every shot. Following the established procedure for cartesian coordinate systems, shots to the left of the aiming point take a negative distance on the x-axis, and shots below the aiming point take a negative distance on the y-axis. You end up with the coordinates of every shot location on a two-dimensional plane.

With that information, you're set up to really evaluate what your gun is doing. The group center is simply the average of the x-axis measurements and the average of the y-axis measurements. So, for example, if you end up with (1.5 inches, -.5 inches) then you know that your gun is shooting, on average and with no guessing, 1.5 inches to the right and a half inch to the left. Using that information you can adjust your sights exactly.

Then, using the Pythagorean theorem, you can calculate the distance of every shot from the group center that you just determined. To do that, you simply subtract the x-axis position of the group center from that of the shot in question, then do the same for the y-axis position, square both quantities and add them together, and then take the square root. All very easy to do with a calculator or, better still, if you set up an Excel spreadsheet.

Once you have the deviation from center for each shot, you can calculate the average deviation, which is a much better evaluation of accuracy than group size, and suffers from none of the problems that I laid out earlier. And, although it's a bit beyond this discussion, using some basic statistics, you can use the standard deviation of the group of deviations from center to determine the probability of striking any size target at any range.
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Old May 26, 2009, 08:55 AM   #16
olmossbak
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FlyFish,

I ask you for the time, not how to build a watch.

PSP,

That's more like it.
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Old May 26, 2009, 10:01 AM   #17
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+1 on center to center
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Old May 26, 2009, 12:37 PM   #18
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Quote:
Typical group measurement is center-to-center of the two holes farthest apart in the group.
This is correct.

Traditionally, meaning the custom began many decades back, folks would shoot 5 rounds. The best 3 of the 5 will tell you what the gun and ammo can do. The total of the 5 let you know what the gun, the ammo and the shooter can do. This means that usually 2 of 5 rounds will be tossed a bit by the shooter or the gun and ammo.

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Old May 26, 2009, 07:20 PM   #19
bulldog1
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Gentelmen

This is a great forum ,the debate is healthy and very informational.

So to get my information i will do the 5 shot group. I will use a target that measures 5.5 in (lf to rt & top to bottom) with a center ring measured the same way of 3.0 inches. I will measure the best 3 of 5 shots that land the farthest to the left to the farthest to the right. I want to gauge my ability in the correct way. I am just an average shooter seeking knowledge. My goal is to place as many shots as possible in the 3in ring with slow or rapid fire when I'm shooting.
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Old May 26, 2009, 08:20 PM   #20
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If you want to measure a group correctly, you'll forget about the "Ayoob" method of not counting "flyers". Measure the center-to-center of the two shots farthest from each other in the five-shot group. What the heck's the use of firing five shots for groups, if you're going to "cherry-pick" the best three??
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Old May 26, 2009, 08:51 PM   #21
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Quote:
What the heck's the use of firing five shots for groups, if you're going to "cherry-pick" the best three??
Well if you are shooting bullseye they all count.

If you are shooting to try out a new gun or load the 3 of 5 method helps. But it helps only if you keep track of both. You count your flyers, but they count as your flyers. The point of the 3 and 5 is to isolate the shooters error from what the gun/ammo can do. You look for consistency. It's one tool in the tool box.

By the way, this method predates Ayoob. He borrowed it, I believe, from bench rest rifle shooting and the methods used to test rifle accuracy at 100 yards and beyond.

Personally, I never shoot for groups. I shoot to hit a target and aim to place each shot where I want it. Groups are the result.

If I comment on a group I've shot I always say "5 rounds in 3.5 inches, 3 of which were inside of 2 inches" or " A good 2 inch group spoilt by my two flyers that opened it to 4 inches". If I were to only mention the 3 best shots it would give a distorted impression of my own abilities something I have no need to do. But citing the 3 best gives someone a better idea of what the gun and ammo can do, IMHO.

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Old May 27, 2009, 12:47 AM   #22
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Flyfish,

Your points about why traditional group size isn't an ideal measurement are all on target. (Sorry 'bout that. )

However, coming up with your own method for measuring groups is also problematic because no one else can compare their results to yours or vice versa.

The military has developed a few standards (CEP, R95, RMS, etc.) for measuring accuracy that answer your objections. Unfortunately, none of them are anywhere near as easy to use as a simple group size measurement.
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Old May 27, 2009, 07:46 AM   #23
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John, thank you for the reference to those military standards. I'll track them down. I did see one or the other of them discussed in one of the AR forums a while back, and it sounded very similar to the method I developed independently, although somewhat more sophisticated (no surprise there).
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Old May 27, 2009, 08:00 AM   #24
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Not to be argumentative, but I'm still of the opinion that the purpose of a five-shot group is not just to test the shooter, but the rifle, as well. When I say I've shot a .226", five-shot group @ 100 yds. with my rifle, the .226" measurement would include the "flyers". Shooting five shots, then throwing out the two worst doesn't test either the rifle or the shooter, as there's no empirical method of determining whether the rifle or the shooter's responsible for the "flyers". Why best three? Why not throw out three and count the best two??
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Old May 27, 2009, 08:50 AM   #25
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I always thought the test of a handguns accuracy was taking into account all the ammo it holds, and find your grouping for that. So if you have a 6 shot revolver, how close are all 6 shots, and if you have a 15 round 9mm how close are the 15 rounds. Maybe others will think it is dumb, but that is the method I measure from. I am not a great shot, so I just tell everyone I am cheap and like to be able to reuse my targets. So I just measure the dents in the sandbank--yardstick works very well.
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