How do you measure groups?

[mljdeckard]

I get that MOA is a geometric measurement which equals 1" and a tiny bit of change @100 yards. If a group fits under a quarter, it's MOA. If it's less than half that, it's about 1/2 MOA.

But how do you measure to get precise numbers? From the edges of the holes? From one outer edge to another minus 1x the diameter of the bullet used? What's the correct way to do it?

I am finally getting some rifles and handloads worth measuring.

 

[Gtscotty]

I measure from the opposite edges of the farthest holes and subtract a caliber, but I'm no pro.

 

[dprice3844444]

very carefully,usually with a ruler

 

[Bwana John]

Why I go by the very best 3 shot group the gun ever fired, then subtract ~ 3/4 moa, doesnt everybody?


Average of 5, 5 shot groups; groups measured from outside edge to outside edge, then subtract one caliber.

 

[tahunua001]

center to center, not edge to edge.

take for instance a 22 caliber vs. a 45. the 45 holes are twice the size as the 22 so a holes touching group with a 22 is half the size as a holes touching group with a 45. that is why when you are measuring accuracy, you must measure from the center of the holes not the edge.

 

[Caliper_Mi]

center to center, not edge to edge.

But, it is often hard to locate the center of the hole. Thus, measuring edge-to-edge, then subtracting the caliber (1/2 the caliber for the hole on the left, plus 1/2 the caliber for the hole on the right) is often easier.

 

[mljdeckard]

Exactly. How do you find the center when you are measuring to .001"?

 

[mljdeckard]

Or when your group is so tight that the circles aren't distinct?

 

[Bwana John]

take for instance a 22 caliber vs. a 45. the 45 holes are twice the size as the 22 so a holes touching group with a 22 is half the size as a holes touching group with a 45. that is why when you are measuring accuracy, you must measure from the center of the holes not the edge.

That is why after measuring the outside to outside You subtract one bullet diameter (caliber) the group was shot with

Take for instance 2 .22 caliber holes that barely touch (.22"+.22"-.22"=.44 group.)

Now do the same for .45, (.45"+.45"-.45"=.90" group)

If you measure from the center to center you will get the same thing, but where is the center?

 

[Bwana John]

Or when your group is so tight that the circles aren't distinct?

Use the bullet on a unfired cartridge pushed into the hole in the paper to find the real edges.

 

[Furncliff]

I can't use this program since it is PC only but give it a try....

http://www.ontargetshooting.com/

 

[Walkalong]

Draw a line between the two bullet holes furthest apart going through their center. Measure the outer edges at the line. If it is a tight group you may need to draw more than one line and measure both to see which is largest.


Pic 1 Link


Pic 2 Link

To get really good measurements use something like this modified caliper.

Here is an article about using it.

 

[mljdeckard]

But Walkalong, it's still minus one caliber, right?

 

[jerkface11]

The acceptable way to measure groups for the internet is to throw a quarter onto the target and take a picture of it. Never under any circumstances mention the range the target was shot.

 

[Warp]

I don't get too fancy or technical with it. I'm just not into that kind of shooting. I don't do bench rest or real-precision stuff...so I don't have to worry about it too much.

I'm good with measuring the farthest holes, far edge to near edge, and calling that the group size.

 

[MCMXI]

I've seen numerous groups on this and other forums measured incorrectly. I use OnTarget every time, all the time. Way too many bullets make holes in paper considerably smaller than their diameter so all of this edge to edge minus a caliber is useless.

 

[mljdeckard]

Ok 1858, how does ontarget work?

 

[mljdeckard]

Never mind, found it.

 

[jr_roosa]

I stopped measuring groups, and if I care I use one of the software apps that does statistical analysis on the vertical and horizontal spreads of the bullets in the group. Measuring the distance between the two holes furthest away from each other doesn't tell me much...I want to know how the whole group performed, so I want to know the average distance from the center of the group, so that one flyer in a ten shot group doesn't confuse the issue.

Overall, I follow trends in scores from matches to see how my rifle and ammo are performing. I look at the numbers of Xs, 10s, 9s, etc. and the trends over time.

-J.

 

[JohnKSa]

Use the bullet on a unfired cartridge pushed into the hole in the paper to find the real edges.

YES. If you're going to use the "edge-to-edge subtract caliber", you must find the REAL edges.

As 1858 correctly points out, it's not unusual to find that the bullet holes in the paper are not actually as big as the bullet that went through them.

It's been my experience that it's common get a more accurate group measurement by "eyeballing" the center of the holes and measuring center-to-center than by assuming that the bullet holes are actual caliber in size and using the edge-to-edge & subtract caliber approach.

An alternative approach is to actually MEASURE the bullet hole diameters in the target and then subtract that value from the edge-to-edge measurement instead of using the nominal caliber figure.

 

[ArthurDent]

Like many folks have said: I measure extreme spread. (There are other ways, such as plotting coordinates for each impact and computing variances... in other words, a lot more work!)

I bought a pair of calipers for the job... it makes things ever so much easier.

1) Measure from the extreme outside edge of the most widely-spaced projectile holes. With the calipers it is much easier to try all possible ways, and to get more digits of accuracy.
2) Subtract one diameter of a round. The answer will give you center-to-center measurements, despite calibre.

As JohnKSa just pointed out, it is a good idea to measure a single, isolated hole in your target. The apparent size may differ from the stated calibre. You should also repeat this measurement for each new type of target paper, 'cause things change depending on the paper.

Now... here's a major factor to consider: How Many Rounds are you going to include in each group???

The more rounds you include in each group, the smaller the variance will be between groups. Unfortunately, the group size will increase if you include more rounds in each group.

I normally use 5 rounds per group. That seems to be a widely accepted number, and it is a LOT better than 3 rounds per group.

Unfortunately, my math says that your group sizes will vary greatly, even under identical conditions. For a 5-round group, and a 95 % confidence interval (look it up if you need to), you can expect your groups to vary over a range of about * 4 *! That's right... do the same exact thing twice, and you could end up with 2 different group sizes... one of 'em * 4 * times bigger than the other one!!! (actually, the difference could be worse... that's just the 95% confidence limit.) I've seen this kind of thing happen, which is one reason I started doing the math.

The way to get more consistency in your numbers is to send lots of rounds down-range and do averaging. I've done some math on this, and I've concluded that it's the total number of rounds that matters. Lots of groups with a small number of rounds each seems to be almost exactly as accurate as a few groups with lots of rounds each.


Now... for a really good time, start discussing the merits of MIL vs MOA measurements.

 

[Warp]

Ahhhh, THAT topic. Unavoidable.

Personally I don't count a group size unless there are at least 5 rounds.

I shoot 3 round "groups" sometimes for confirming zero or figuring out where a rifle shoots at different distances, to conserve ammo.

 

[Bart B.]

Arsenals in the USA have traditionally shot several dozen shots of rifle ammo at 600 yards per test group. Then calculate the mean radius of all the shot holes. I've seen some of those 200+ shot groups fired at 600 yards measuring about 7 inches extreme spread for match ammo. Service ammo's three times that size. Mean radius is probably the best statistical method as it gives the average miss distance a load will have when the point of aim's the center of the group.

There's a popular method of shooting a few 3- or 5- shot groups, then taking the smallest one to be the "accuracy" a given load shoots. Most folks don't realize that the smallest groups are made when all the variables cancel each other out. When the variables all add up together, the largest group's at hand.

Averaging the extreme spread of several few-shot groups tells you that 2/3rds of all groups shot will be no bigger than that size. But 1/3rd will be bigger. If all those shots were fired on the same target (example, four 5-shot groups' total of 20 shots), the extreme spread will be bigger than the largest few-shot group. And the center of each 5-shot string will not be at the same place relative to the aiming point.

 

[spottedpony]

That is why after measuring the outside to outside You subtract one bullet diameter (caliber) the group was shot with

Take for instance 2 .22 caliber holes that barely touch (.22"+.22"-.22"=.44 group.)

Now do the same for .45, (.45"+.45"-.45"=.90" group)

If you measure from the center to center you will get the same thing, but where is the center?

shouldnt that be .22+.22=.44-.22=.22? or .45+.45=.90-.45=.45?

 

[Walkalong]

But Walkalong, it's still minus one caliber, right?

Yes.

I've seen numerous groups on this and other forums measured incorrectly.

Numerous may be too kind.


Unless you are shooting in a registered match of some sort, or are working up a load for such endeavors, getting the group size exactly right is not really a big deal IMO.