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View Full Version : How do you measure your shot groups?

Palmetto-Pride
May 4, 2010, 04:00 PM
So I have in the past always measured the distance between the two furthest bullet holes and considered that distance what my rifle/ammo is MOA capable.....right?

Well hear me out for a second I have another way to look at it. Lets say your shooting a 5 shot group at a dot on a piece of paper at 100yds and for every shot your POA was exactly the same(the center of the dot) now you check your group, you have 3 holes pretty much in the middle then you have one hole 3/4" away from the center at say 12 O' clock and another one 3/4" away from the center at say 6 O' clock so your furthest holes are 1.5" away, but none of your holes are ever more than 3/4" away from your POA (point of aim) So do any of you still feel good about your shooting even though your groups are 1.5", but your POA and POI are never more than 3/4" different? Sorry I know I took the long way around on this....lol

ScottRiqui
May 4, 2010, 04:16 PM
I see what you're getting at, and you're correct - not all "1.5 inch" groups are created equal. I'd feel better about a group that has a cloverleaf at the POA and a 1.5" "flyer" than I would about a group with all the shots evenly distributed within a 1.5" circle, but they'd both be 1.5-inch groups.

But there's no way to completely define the quality of a grouping with a single number, so we use what we have.

If you wanted a better number (with a little more work), you could measure each hole's distance from the POA, add up all the distances and divide by the number of shots, but even that allows for some ambiguity, since a tight cluster of shots 2" from the POA would "count" the same as group where all of the shots are 2" from the POA, but in different directions.

And really, what are you looking to evaluate - accuracy or consistency? Is a cloverleaf 2" away from the POA better than a 2" group centered around the POA?

Once you've decided what's important to you in your groups, you can apply as much math as you'd like to come up with a way to consistently evaluate your groups.

jmr40
May 4, 2010, 04:18 PM
You are still shooting 1.5" groups either way. Truthfully that is plenty good enough for 95% of the hunting we do and we all sometimes worry too much about having a gun that will shoot 1/2" groups. I'm as guilty as anyone.

Given a choice between a 1.5" rifle that I really liked and that fit me well and a .5" rifle that weighed a ton and balanced poorly, I'd hunt with the 1.5" gun.

But I'd rather have a well fitting, well balanced lightweight rifle that would still shoot .5" groups.

wogpotter
May 4, 2010, 04:53 PM
pick furthest apart holes.
either:
Measure across outsides & deduct caliber.
or:
Measure from inside of one to outside of other.

Either way it creates a center-to-center group size.

ScottRiqui
May 4, 2010, 04:58 PM
pick furthest apart holes.
either:
Measure across outsides & deduct caliber.
or:
Measure from inside of one to outside of other.

Either way it creates a center-to-center group size.

That's certainly the most-common way to measure group size, but I think the point the OP was getting at was that this method can give equal "scores" to two groups that are fundamentally different. And it really only measures consistency and not accuracy, since the distance between the holes and the POA is ignored completely. Of course, the assumption there is that if you have consistency, problems with accuracy can be fixed by either adjusting the sights or the shooter's habits.

Palmetto-Pride
May 4, 2010, 05:24 PM
Truthfully that is plenty good enough for 95% of the hunting we do and we all sometimes worry too much about having a gun that will shoot 1/2" groups. I'm as guilty as anyone.

You are totally correct, but I find myself obsessing about it, I want the bullet to go where I aim each and every time, but I know that's not realistic......:)

ScottRiqui
May 4, 2010, 05:49 PM
If you really want measurements that give you the most information about your groups, here's what I'd do:

Shoot at a piece of graph paper with a small target spot on it.

Label the bottom-left corner of the paper (0,0) and increase the numbers as you move right and up. So a shot that lands in the 21st column from the left and the 15th row from the bottom would be (21,15). Repeat for all of your shots (I'll use 5 shots for my example).

Add up the all of the x-values (the first number for each shot) and divide by five. Then add up the y-values (the second number for each shot) and divide that by five. Now you'll have the x- and y-coordinates of the geometric center of your group.

Now, mark that spot on your target and measure the distance of each shot from the center. Add up those distances and divide by five. That's the average distance that each shot is from the center of the group, and smaller is better. You could also take those five distances and figure out the standard deviation (again, smaller is better.)

This wouldn't be too much work with a spreadsheet, and the results you get would clearly differentiate between a group with four tight shots and a flyer compared to five more loosely-spaced shots, even if the traditionally-measured group size would be the same for both groups.

Note that this method still ignores the distance between the POI and POA - if you're consistent enough to be obsessing over your groups, you pretty much have to write off POI/POA differences as sighting issues.

Buzzcook
May 4, 2010, 05:53 PM
Well what if all holes group within .5" but that group is 2" from the poa. Do you then say you have a 2.25"group?

Grouping is more about rifle/bullet performance than about accuracy compared to a fixed point.

It's the difference between accuracy and precision.

Old Grump
May 4, 2010, 05:58 PM
Well what if all holes group within .5" but that group is 2" from the poa. Do you then say you have a 2.25"group?
.5" group which needs a 2" sight adjustment. Shooting is good, gun is good, ammo is good.

Palmetto-Pride
May 4, 2010, 05:59 PM
I guess thats really my point do most people worry more about a tight group or how close that group is to the POA, I worry more about my POA and POI being as close as possible.......

ScottRiqui
May 4, 2010, 06:06 PM
I think most people are concerned with how tight the group is, because if you can consistently shoot tight groups, matching POI to POA is usually just a matter of adjusting the sights.

Group size is also a better measurement of the rifle itself. Any rifle can be adjusted to hit the POA (or at least to center its groups around the POA). Not every rifle/ammo combo is capable of shooting an extremely tight group, no matter how skilled the shooter.

kaylorinhi
May 4, 2010, 06:15 PM
I understand all the above but know that if I messed up "that" shot, I will not count it against my rifle. I will also count "that" shot if I know I did it right, and that is about as precise as I get, my gun probably shoots better than me anyway. I will start using the "out of one, in of another" method as the complete and utter lack of math is a good thing for an old professional bullet-sponge!

Cosmik de Bris
May 4, 2010, 07:20 PM
Have a look at this, this guy does some analysis on grouping and it is quite interesting.

http://www.rifleman.org.uk/Fuller_group_diagnosis.htm

Abel
May 4, 2010, 07:58 PM
I have always gone center to center on the two furthest holes of five shots. That's as simple as it gets.

slab11
May 4, 2010, 08:24 PM
If you go to 6mmbr.com you can download OnTarget software for free and if you have a scanner or digital camera, you can download targets and with a little piddling, can measure groups fairly accurately. It will calculate everything for you.
see example

bamaranger
May 5, 2010, 01:06 AM
I read somewhere that Berdan's Sharphooters, of Civil War era, shot and measured groups buy a "string" method, where each shot was measured on a string from a center mark and a run of 5-10 shots could not exceed a certain string length (total?) to qualify for the unit.

Don't mean to hijack thread, but its another "group measuring technique" of which I've heard.

wogpotter
May 5, 2010, 08:27 AM
I thought the OP was asking about group size, not "score", or POI.
All three are checked differently IMHO.

To find POI measure either outside-to-outside, or inside-to-inside along longest & shortest axis. Mark intersection of lines, this is your POI.

For score add up the numbers on the target rings, this is your score.

There are even two ways of doing this.
Inside scoring: if the bullet hole breaks a line you get the higher score.
Outside scoring: If the bullet hole brakes the line you get the lower score.

ScottRiqui
May 5, 2010, 09:11 AM
I think the OP had a pretty good grasp on the common method of measuring group size right from the start. His post just brought up the fact that a single diameter measurement isn't always very descriptive of the "quality" of a group, and that it ignores the relationship between POA and POI.

PetahW
May 5, 2010, 10:15 AM
Here ya go : http://www.shootingsoftware.com/measure.htm

.

Abel
May 5, 2010, 04:19 PM
Yall are seriously overthinking this. Just b/c the bullet has strayed only 1.5" from the POA doesn't mean your gun is 1.5 MOA. The bullet's POI's are measured in relation to each other, not your POA.

44 AMP
May 5, 2010, 08:21 PM
Measure maximum distance, either center to center or outside to outside (deducting bullet diameter). This is your group size. Relationship to point of aim has nothing to do with group size.

Every wonder why a lot of those groups they show in magazines aren't centered on the target? Because that is another step, and one not needed to determine the accuracy of a given load. When doing a lot of shooting testing groups, and since different loads can have different points of impact, relative to the aiming point, we don't bother to waste time or ammo zeroing each load (matching POI with POA). We do that when we settle on which load we are going to use for serious shooting.

Centering your group on the point of aim is done for hunting, and for target shooting for score, where it is important. But not for just testing and determining group size.

And, remember, to take group size and cut it in half when figuring how far off point of aim any particular shot will be, once you do sight in.

For example, a 1.5" group, means that (when sighted in) that any individual bullet strike should be no more than .75" away from the aiming point. But it can be that much off in any direction.

4 shots in one hole, and one shot 1.5" away (when its not you, or the wind, but the gun/ammo doing that) is still a 1.5" group. Called flyers (before looking at where they hit) are not counted.

Palmetto-Pride
May 7, 2010, 08:53 AM
http://i175.photobucket.com/albums/w155/bojack2575/071ff7ba.jpg

This is kind of like the group I am talking about. Even though the over all spread is 1.5" I am never more than 3/4" away from my POA. I feel it's not too bad for a 16" AR-15 @ 100yds.

L Puckett
May 7, 2010, 10:36 AM
1 1/2 inch center to center (or outside minus dia) is an 1.5 group. Whether it is caused by barrel oscillation, shooter or ammunition that is the reference for group size.

Point of impact VS point of aim is as stated above, a simple (usually) adjustment of the sight system. Benchrest shooters use a differnt point of aim V POI on purpose. A rifleman goes to the X.

As for your 1.5 group at 100 yrds.....nice shooting. A production AR shooting LC 55 grn producing a 1.5 at 100 yrds requires a good shooter. Now if your AR is a custom with a match barrel that is free floated and has a high end trigger shooting tuned match ammunition, then an 1.5 would indicate something needs work.

Again, nice shooting,
LP

Palmetto-Pride
May 7, 2010, 12:34 PM
Thanks, no it's a Smitih & Wesson M&P15T using Hornady 75gr TAP ammo.

L Puckett
May 7, 2010, 01:09 PM
Very nice rifle without a doubt, but you still shot a good group. Your last pic shows horizontal spread, interesting. I would look at 1) Wind 2) Trigger 3) Natural Point of Aim and consistant cheekweld in an attept to reduce group size. There are several other issues too, but in an AR I see these generate improvement in more cases with horizontal spread.

I'd probably first look at an improved trigger (FCG) and I have seen TAP ammo run good and bad depending on the rifle. Just for the sake of discussion I'd try a box of 69 grn Match and a box of 53 grn Match in that 1:9 barrel.

Still, a 1.5 group would make a bunch of guys say "I hope I never make him mad". LOL nice shooting.

LP

Palmetto-Pride
May 7, 2010, 04:58 PM