How do you measure your group sizes? I was always told "center from center" of the holes...

Outside measurement of greatest "spread" (farthest apart holes), then deduct the bullet diameter. The results are the same as the center to center method.

Center to center....outside to ouside subtract hole dia.....outside farthest hole to inside farthest hole.....all the same results.

The most common method is to eyeball it, give it your best guess, and then subtract 3/4".

quote:

Originally posted by duckboat:
The most common method is to eyeball it, give it your best guess, and then subtract 3/4".

And subtract another 1/4", and then post on your favorite forum that your rifles will shoot 1/2" groups all day long "as long as I do my part".

Don't worry about measurements, just come up with a sub MOA number that you think will impress people, then post on a forum.

mailman

Seriously - there are two good ways to measure groups. Assuming that you are interested in comparing groups for purposes of deciding the best loads.

For short distances (less than 500 yards)- center to center of the two widest shots.

For long distances (500 yards +) - mean radius average.

Ray

A common way it is done is to measure from outside edge to opposite outside edge of the two farthest apart shots, then deduct bullet diameter.

That, however will not give a correct center-to-center measurement.

One needs to deduct not the bullet diameter, but the diameter of 1 hole made by that same bullet on that same target paper. Such a bullet hole is almost invariably smaller than the diameter of the bullet being shot.

So, if one deducts the full bullet-diameter, they will get a figure which is slightly smaller than their actual group size.

For instance, .308 bullets frequently make holes less than .300" diameter. So, if one measures from outside to outside of two .300" holes, the resulting figure will be .008" smaller than if the holes were a full .308" diameter.

That is actually a very conservative estimate of the usual error. It usually runs somthing more like .015" to .025" or even more error.

Ted Thorne got it right, above.


A method which is not common now, but used to be very common, is "string" measurement. To measure the "string"size of a group, a pin is stuck at the exact center of the point of aim. Then a closely discriminating tool such as a caliper is used to measure the distance from the pin to the center of each bullet hole in the group.

All those center of target to center of bullet hole figures are added up, giving the total string length. What that tells the shooter is the total distance from the point of aim all of the shots landed. If the shooter wants then to know the average distance from the center of the point of aim, he/she simply divides by the number of shots fired.

For determining true accuracy of my rifles, I prefer the string method. The load or rifle with the shortest average string length per shot is the one which dependably hit closest to center of aim.

AC
All methiods have issues. With "String" a honest 1 hole group (lets say a .3 x.3" hole made by 5 .223 bullets) that's 6" away from the aim point is worse than a 4" group that's centered on the aiming point. I can adjust the sights to bring the "one holer" group into the aim point, but the 4" group is as good as it's going to get (as far as sighting is concerned)

posting 1/2 inch groups on the forum won't kill game if it's for EGO purposes.

quote:

A common way it is done is to measure from outside edge to opposite outside edge of the two farthest apart shots, then deduct bullet diameter.

That, however will not give a correct center-to-center measurement.

One needs to deduct not the bullet diameter, but the diameter of 1 hole made by that same bullet on that same target paper. Such a bullet hole is almost invariably smaller than the diameter of the bullet being shot.

If you take the two further apart holes and measure from the outside of one hole to the inside of the other or vice versa, then you eliminate the factor of the hole being smaller than the bullet that made it.

Also, you shouldn't be measuring the hole diameter but the edge of the stain on the paper where the side of the bullet rubbed whilst traversing the paper.

I'm continually hearing about these 1/2" sporters and 1/4" varmint rifles. I always invite these guys out to the range to shoot. Funny thing though, they never show up.

quote:

Originally posted by Alberta Canuck:
...A method which is not common now, but used to be very common, is "string" measurement. To measure the "string"size of a group, a pin is stuck at the exact center of the point of aim. Then a closely discriminating tool such as a caliper is used to measure the distance from the pin to the center of each bullet hole in the group.

All those center of target to center of bullet hole figures are added up, giving the total string length. What that tells the shooter is the total distance from the point of aim all of the shots landed. If the shooter wants then to know the average distance from the center of the point of aim, he/she simply divides by the number of shots fired. ....

Hey AC, How `bout the "diameter" of that HUGE PIN???

Don't you have to subtract 1/2 the diameter of the Pin?

How do you find the Center of the Holes that go through the other Holes without the rolling Witness Paper Backer, if you plan to Average them? One shot per Target?

How do you determine where the "Center of the Hole is" - with a Caliper - unless you measure from an edge?

Hey AC, Looks like you "might" be shooting some amazing groups and not accounting for the Pin diameter.
-----

How can a Bullet cut a smaller Hole than it's own diameter?

I have a 7mmRemMag which will overlap 2-shots using Partitions. Have told a number of people I'd better not shoot a 3rd shot into the group because it shoots so tight the Hole would simply close back up. Maybe the smaller Hole Theory is what would cause that to happen.
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I need to go do some measuring.

Outside to outside subtracting caliber. A yard stick is usually sufficient to measure my groups .

Ken....

Hey AC, I got to do some measuring today. For the most part, you were absolutely correct about the Bullet Holes being smaller than the Bullet Diameters.

Bullet Holes shot with a 223Rem, 243Win, 7mm-08, 7mmRemMag and a 308Win were all smaller than the actual Bullet Diameter. Only exception I saw was the 0.429" holes from the 444Mar.

Didn't check the 350RemMag, revolver or pistol holes.

So, why is the Bullet Hole smaller than the Bullet?

I did notice the "apparent"Hole going through the Black part of the Target(plain 8.5"x11" IJP Paper) measures Larger than what I can see going through the White portion.

I don't think I have any of the heavier regulation Target Paper. But if I come across any, I will try it out.

Anyone know what is going on?

I used too have a circle template used in drafting.....left over from the old days before CADD...It makes a great tool to sit on the group and find the "best fit"...I recommend one if you can find one......today a 6" scale gets the job done as I'm not at all a benchrest shooter. (center to center and estimate)

Yes at times the 6" scale isn't enough.....but then I don't really want to measure it anyway in that case.

quote:

Originally posted by Hot Core:
Hey AC, I got to do some measuring today. For the most part, you were absolutely correct about the Bullet Holes being smaller than the Bullet Diameters.

Bullet Holes shot with a 223Rem, 243Win, 7mm-08, 7mmRemMag and a 308Win were all smaller than the actual Bullet Diameter. Only exception I saw was the 0.429" holes from the 444Mar.

Didn't check the 350RemMag, revolver or pistol holes.

So, why is the Bullet Hole smaller than the Bullet?

I did notice the "apparent"Hole going through the Black part of the Target(plain 8.5"x11" IJP Paper) measures Larger than what I can see going through the White portion.

I don't think I have any of the heavier regulation Target Paper. But if I come across any, I will try it out.

Anyone know what is going on?

I'd guess the hole is somewhat smaller than the actual bullet diameter due to the bullet being tapered and maybe target paper "springback"? I haven't ever checked, but I bet holes made by a wadcutter bullet are probably real close to bullet diameter.

That's true using most paper targets. Match targets are a bit differently constructed. Caldwell makes a line of BR score or practice targets (tip top targets) that will give you a true diameter. as a little bonus for my area, they are also waterproof. Other people make match similar targets I happen to have the Caldwells' on hand and as I recall they were a bit less expensive.

The trouble with center to center measure and the reason it's not used much for match scoring is that it gets into judgment calls as to where the center actually is. This can lead to some real controversy if you have a group comprised of something like two discrete shots and a glob of three all practically in the same place. At one venue here they have dropped even the use of calipers in favor of a special tool to measure groups.

To get a general idea of how your rifle is shooting, just for your own info, the caliper and furthest edge method on paper targets is fine. If you're getting into a competition with your buddy about groups, you ought to do it at a real match. You'd be surprised at how much you can learn there and have fun too. And there is sure to be someone who will impartially measure your groups.

quote:

Originally posted by Grumulkin:

[ ... ]

If you take the two further apart holes and measure from the outside of one hole to the inside of the other or vice versa, then you eliminate the factor of the hole being smaller than the bullet that made it.

Also, you shouldn't be measuring the hole diameter but the edge of the stain on the paper where the side of the bullet rubbed whilst traversing the paper.

Exactly. Well said.

quote:

Originally posted by Grumulkin:

quote:

A common way it is done is to measure from outside edge to opposite outside edge of the two farthest apart shots, then deduct bullet diameter.

That, however will not give a correct center-to-center measurement.

One needs to deduct not the bullet diameter, but the diameter of 1 hole made by that same bullet on that same target paper. Such a bullet hole is almost invariably smaller than the diameter of the bullet being shot.

If you take the two further apart holes and measure from the outside of one hole to the inside of the other or vice versa, then you eliminate the factor of the hole being smaller than the bullet that made it.

Also, you shouldn't be measuring the hole diameter but the edge of the stain on the paper where the side of the bullet rubbed whilst traversing the paper.

You are correct about measuring the edge of the stain. That's what I meant, but didn't state correctly. I was counting whatever people usually see and measure from as "hole",but actually, I DO measure from stain edge to stain edge.

Even the stain edges, though, are often smaller than the actual bullet diameter. Paper is made to stretch slightly, otherwise it would be so crisp as to be much less useful for most of its purposes. That allows the bullet to pass through while both stretching and cutting the paper, leaving a smaller hole. Official "target" paper does so less than most other papers, but it still stretches to a small extent before the bullet "bursts" through.

Wadcutters DO cut a hole closer to actual bullet diameter, but few riflemen I know personally shoot wadcutters.

The reason I don't advocate measuring from the outside of one extreme hole to the inside of the other extreme hole is that the calipers usually cover much or all of one of the holes that way...thus making the measurement somewhat less than exact. (The calipers do't need to cover any of either of the holes, but that is the way most people I have observed measuring targets apply them.) And, I've noticed over the years of checking group sizes at matches that humans have a tendency to be inexact in the "smaller" direction than in the "larger" direction.

And, of course one of the weaknesses of the string method is that if one is shooting tiny one-hole groups, they can't identify the separate holes in the groups. Because we don't live in heavily settled areas out here, the easy answer for checking accuracy with string measurements is to just shoot at targets farther away...like at 400 or 500 yards where darned few groups are one-holers.

Hi HotCore - Well, anyone who can measure a group can probably measure the diameter of a pin, too...then if they want to allow for the pin, they just subtract half its diameter from the final string length. In any given match that isn't necessary, as the scorer will use the same pin for each group, thus group sizes will all contain the same "pin error".

quote:

Originally posted by Alberta Canuck:
...Hi HotCore - Well, anyone who can measure a group can probably measure the diameter of a pin, too...then if they want to allow for the pin, they just subtract half its diameter from the final string length. In any given match that isn't necessary, as the scorer will use the same pin for each group, thus group sizes will all contain the same "pin error".

You have gotten to the point I'm at when measuring my groups. Probably does contain some Error because I'm not measuring with a precise measuring instrument like a Visual Comparitor. But it doesn't matter because I'm normally only shooting against "me".

Had not realized about the Holes being smaller in certain types of Paper. Don't want to change now, because all my historical data would need to be remeasured.

I did find some Targets I'd made for l-o-n-g distance with Construction Paper and they seem to be fairly close to Bullet Diameter.

Sure appreciate you mentioning it though. SHould be good for a few FREE BBQ Suppers by just mentioning it at the Range and seeing who cares to argue.

quote:

Originally posted by Tailgunner:
AC
All methiods have issues. With "String" a honest 1 hole group (lets say a .3 x.3" hole made by 5 .223 bullets) that's 6" away from the aim point is worse than a 4" group that's centered on the aiming point. I can adjust the sights to bring the "one holer" group into the aim point, but the 4" group is as good as it's going to get (as far as sighting is concerned)

Depends on what you're trying to learn, Tailgunner. Grouping ability by itself is useless unless the group IS centered on the point of aim.

Little tiny groups may tell you how consistently the bullet struck close together on a given target, but they tell you nothing at all about how well they hit relative to point of aim. By measuring the average distance each bullet strikes from a given point of aim, you learn how accurate your rifle really is...that is, how likely you are to hit a particular item being shot at, without having to adjust point of impact. In many of the old-time matches, the shooters were allowed NO sighting shots, therefore no adjustments would enable shooters to usually win the matches unless their very first shot was on the point of aim.

It is also why Camp Perry is a "score" shoot rather than a "group" shoot. The Perry matches grew out of a program which was intended to teach shooters to hit enemy soldiers. A tiny group of misses was not a desirable outcome. Scoring rings are a method of approximate "string" measurement which does not slow the match progress down by all of the math of an exact string measurement.

For hunters, having a consistent point of impact right at or very close to point of aim is also the name of the game.

A .3 x .3 group of 5 shots which are six inches from the point of aim will miss an upright prairie dog every single shot, even at 100 yards. A 4" string of 5 shots means every single shot struck an average of 0.8" from point of aim. That's probably gonna hit that prairie dog every shot at the same 100 yards, and some of the time even at 300 yards, while the group that was 6" off at 100 yards will be 18" off at 300 yards.

Of course, the best of all for the hunter would possibly be to shoot one shot from a cold barrel every time he went to the range, measure the distance of each shot from point of aim, and then average the results for all those shots over time. That would tell him if his rfle consistently shoots to point of aim, or if not, how much it misses by. For me that is much more useful information now that I no longer care about benchrest match shooting..

quote:

... one shot from a cold barrel every time he went to the range, ... and then average the results for all those shots over time.

That gives me an idea - something I have just not thought of before - set up five targets and shoot one shot at each! (In sequence). Mmmm.....

On the topic of measuring group sizes, how reliable is the three shot group in indicating the potential group size and more importantly, the potential POI relative to POA for purposes of making scope adjustments? Say two shots print close and one a bit away or a neat triangle?

quote:

Originally posted by swampshooter:
I'm continually hearing about these 1/2" sporters and 1/4" varmint rifles. I always invite these guys out to the range to shoot. Funny thing though, they never show up.

FIVE shot group, .223 Ackley, at 100 yds from a Rem. 700 PSS. The circle is 1" dia.



I measured it at about 0.22"

Zedo

How about showing us the other groups?

All

There are two measurements of a rifle's suitability for the job. Accuracy and Precision. A group Benchrest rifle must be supremely accurate but not necessarily precise. A hunting rifle must be precise first and accurate second. A Score Benchrest rifle must be both.

Ray

Cheechako-

I sort of agree with you..though I think ANY rifle shot for score has to be both, whether high power score or benchrest score. To do well in a score match, the smaller the group the rifle will consistently deliver, AND the better it consistently holds its "zero" on the point of aim, the better.

That's why I prefer string measurement. It gives you both an idea of average group size and how closely the rifle shoots to zero.

Cheers!

quote:

Originally posted by 303Guy:

quote:

... one shot from a cold barrel every time he went to the range, ... and then average the results for all those shots over time.

That gives me an idea - something I have just not thought of before - set up five targets and shoot one shot at each! (In sequence). Mmmm.....

That's one reason benchrest score matches are fired one shot each at 5 different bullseyes on one large target. Shooter not only has to read the wind, rifle has to hold POI to POA from shot to shot.

On the topic of measuring group sizes, how reliable is the three shot group in indicating the potential group size and more importantly, the potential POI relative to POA for purposes of making scope adjustments? Say two shots print close and one a bit away or a neat triangle?

For doing scope adjustments, 3 shot groups are likely ideal. You figure the distance from the middle of the group fired to where you wanted the bullets to strike. Then you adjust the scope so that the middle of the next group should be exactly where you want the rifle sighted to hit.

Just for shits and giggles:

As long as you're not shooting a 10-shot group the method of measurement is somewhat irrelevant. A three shot group is a "partial" group and maybe and maybe not indicative of the rifle's accuracy.....

If you're really serious about knowing your rifle's accuracy then you should shoot a ten shot group.....to get highly sophisticated about measuring a three shot group is like entering a trivia contest.....fun, yes, but in the end you're no better off than before.

To a large extent Vapo has told the truth in his post just above.

The problem is, one has to determine what level of statistical predicatability of where future shots will land he will be satisfied with.

I have read all kinds of papers by statisticians over the years in respect of how many shots are enough to determine the "accuracy" of a rifle, and why. Some say 10 shot groups are needed, others say 3-shot, 5-shot, or 7-shot groups.

I have reached my own standards through years of applying them to my hunting and competition shooting. For me (and maybe NO ONE ELSE) I see it this way.

For zeroing an already proven accurate hunting rifle, three, three shot groups will do.

For determining if a load is worth further work (tweaking seating depth, trying different primers, etc.) three 5-shot groups will tell me if I want to continue working with a particular combination.

For selecting between two different, excellent, loads to find the absolute best one, five 10-shot groups of each are necessary.

For tweaking my super accurate benchrest rifles' powder charges just before the start of a match, one three-shot group at each powder measure setting is plenty.

I think keeping track of how well your groups tell you what you want to know, will determine how many shots YOUR groups should be.

I KNOW OUR ANCESTORS ALL BELIVED IN 10-SHOT GROUPS AS A MINIMUM. (But they also believed that if man was intended to fly, he would have been born with wings.)

quote:

I KNOW OUR ANCESTORS ALL BELIVED IN 10-SHOT GROUPS AS A MINIMUM. (But they also believed that if man was intended to fly, he would have been born with wings.)

If the Lord had intended us to fly he'd have given us a brain so we could figure out how to do it.....and any other thing as well.....Had he thought it was important to know our group size to within .020 he'd have told Mozus to shoot 10-shot groups and record it in the book Genesis.....

I forget what it's called but we used to use a lexan gauge that was caliber specific. You placed it in the holes amd it had a rod sticking out form dead center and you could measure from tip to tip of the rod sticking out of the lexan button. The lexan also magnified the paper under the button to verify whether or not you broke the ring of the next highest scoring ring.

quote:

I think keeping track of how well your groups tell you what you want to know, will determine how many shots YOUR groups should be.

Thanks Alberta Canuck. That makes perfect sense.

vapodog, that was funny!

quote:

Originally posted by Cheechako:
Zedo

How about showing us the other groups?

All

There are two measurements of a rifle's suitability for the job. Accuracy and Precision. A group Benchrest rifle must be supremely accurate but not necessarily precise. A hunting rifle must be precise first and accurate second. A Score Benchrest rifle must be both.

Ray

I sold that rifle years ago. It was taking me 3 hours to load 20 rds. 5 rd. group, swab the bore, let it cool off.

Much prefer the 8mm Mauser and surplus ammo circa the 1950's -- shooting at stumps from 700 yds.

Some of you guy's would complain if you were hung with a new rope! As long as you are happy with the way you do it, what does it matter. What's it all mean....
Somkin Weed.