How to measure accuracy to 3 decimal places?

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elwaine

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Apologies in advance for this newbie question.

I'm about to start reloading (rifle) and I've been reading as much as I can before venturing forward. I've also been watching experienced reloaders and asking lots of questions - but here's one I haven't asked before: When measuring group size to determine accuracy of the loads you're working up, how does one determine accuracy to a thousandth of an inch?

I see published group sizes listed as something like .337, .873, .566, 1.119. My calipers can measure to that precision but my errors in determining the exact center of the bullet hole in the target exceed 1/1000". Just by looking at my target, I can't truthfully determine exactly where the center of the bullet hole is to anything more than 2 decimal places at best. What's the trick to achieving accuracy measurement to three decimal places? How do the experts measure to an accuracy of 1/1000"?

Thanks in advance...
 
Thank you! As always, it's all in the know how... and I've got a lot to learn.

Best regards...
 
Measure the largest space between bullet holes, outer edge to outer edge, and subtract the caliber.
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I believe it's more accurate and more widely accepted to measure by drawing an imaginary line thru the centers of the the farthest-apart holes and then measuring from the outside rim of one to the same-side outside rim of the other. That way there's no adding, subtracting or wondering if the bullet was partly mushroomed on the way to the target.
 
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The Neil Jones tool is what the BR guys use.
There have been other makers over the years but they are about the same.

It will measure as large as your dial caliper.
12 inch calipers are a little pricey.

Unless you have a BR rifle, just eyeball it.
 
Measuring outside of one hole to the inside of the furthest hole and measuring from outside to outside and subtracting the caliber have equivalent accuracy. I just use calipers to measure the hole distances. If you have a "one hole" group the latter method is the only one that will work.

That said, measuring group size to 3 decimal places is a bit optimistic though I do it myself. That last decimal place is open to a lot of interpretation and error. Do you really think you can distinguish a 1,000th inch difference?
 
I have lost a place in an aggregate by .0001 before. 4th doesn't get any wood to bring home. :(

When it is that close they get the guys who they have the most faith in to measure and remeasure. They have to remeasure 10 groups, your 5 and the other guys 5. :scrutiny: No pressure. :uhoh:
 
When it is that close they get the guys who they have the most faith in to measure and remeasure.
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It's definitely faith and faith alone since no human being can by the eyeball distinguish a 10,000th inch increment on a target whilst applying calipers to measure said minuscule difference. If they are going to pretend to measure such a small difference, they need to be using a microscope.
 
If you are just doing this for your own information. It is ok to Measure from outside to outside and subtract the bullet diameter.

I have never heard of a bullet opening up in the air before. I have seen bullets explode in flight because they were going to fast.

Just my 2 cents worth. I do nto see the need to spend $80 on something that is not really needed.
 
If they are going to pretend to measure such a small difference, they need to be using a microscope.
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They measure each group to the nearest .001, and even measureing that finely is as much an art as a science. Then they average the 5 groups to the nearest .0001 for the aggregate. Their measurements may not be written in stone, but that is how it is done, nonetheless. :)

I have never heard of a bullet opening up in the air before
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I noticed that one too, cpttango30

I do nto see the need to spend $80 on something that is not really needed.
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I do not know anyone who uses one. I have only seen them used at matches, where it counts a wee bit more. :)
 
It's definitely faith and faith alone since no human being can by the eyeball distinguish a 10,000th inch increment on a target whilst applying calipers to measure said minuscule difference. If they are going to pretend to measure such a small difference, they need to be using a microscope.
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You did notice the magnifying lens on the adapter?
Add to that theat you have caliber size circles to center over the holes (or portions) and 0.0001 is not as hard as you would think.

I do not get out the microscope till I need microinches (0.000001) and even then an optical comparator is often used.
It magnifies the image up to a screen for easier measuring.
 
I have never heard of a bullet opening up in the air before. I have seen bullets explode in flight because they were going to fast.
Click to expand...

I saw an article recently; I just wish I remember where. It demonstrated via high speed photography that a soft nosed spitzer bullet becomes more round nosed when shot. It would seem to follow logically that the bullet itself could expand a few ten thousandths of an inch in flight.

Another potential factor could be bullet wobble. If the bullet wasn't traveling perfectly straignt (i.e., perfectly aligned with its central long axis) it would make a hole slightly larger than bullet diameter. This potential error could also make measuring groups to ten thousandths of an inch more a matter of faith than science.
 
wondering if the bullet was partly mushroomed on the way to the target.
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This is what was said. They are not going to mushroom on the way to the target. If they swell a bit, then everybodies is doing it and so are still on the same playing field.

Measuring to that level is not as difficult as it sounds, really, although I understand where you are coming from. There will be the human factor, and thus some error at times. I never worried about it. Sort of like bad calls in baseball. They usually even out, but sometimes they just kill you. It's all part of the game. :)
 
It demonstrated via high speed photography that a soft nosed spitzer bullet becomes more round nosed when shot. It would seem to follow logically that the bullet itself could expand a few ten thousandths of an inch in flight.
Click to expand...

The nose sets back slightly but the barrel maintains the outside diameter.
The only thing that can change is shape and length.

Once acceleration stops the only other forces are aerodynamic drag and centripetal from the spinning.
You can make a lightly built bullet come apart from high RPM.

This is how a bullet obturates (AKA 'slugs up') to create the tight seal needed to contain 50,000 PSI gas.

The ,22 rinfiore BR guys have spent a huge amount of time discovering the soft lead slugs deform from the acceleration in the barrel.
 
Regarding any measurement, the last digit is not significant, period. If measuring to .001, then the measurement is technically significant only to .01. If one measures a group to .001 x 5 times, then averages to .0001, then regarding the calculation itself, the result is only significant to .001. Also, does one realize, just for thoughts sake, that a human hair is about .006, give or take .002 depending on the supplier? I spent a lot of years in the machinest trade, and one cannot differentiate .001 without magnification (or even .002). Measuring something as pliable as the edges of bullet holes through paper, .001 is is a fleeting dream. I know that the bench rest guys do so, but no matter, it ain't accurate. I suppose it's fair to all because the same error potentials exist for all, but it in no way is actually accurate to .001.
 
Measuring something as pliable as the edges of bullet holes through paper, .001 is is a fleeting dream. I know that the bench rest guys do so, but no matter, it ain't accurate. I suppose it's fair to all because the same error potentials exist for all, but it in no way is actually accurate to .001.
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Amen brother!
 
I see published group sizes listed as something like .337, .873, .566, 1.119. My calipers can measure to that precision but my errors in determining the exact center of the bullet hole in the target exceed 1/1000". Just by looking at my target, I can't truthfully determine exactly where the center of the bullet hole is to anything more than 2 decimal places at best. What's the trick to achieving accuracy measurement to three decimal places? How do the experts measure to an accuracy of 1/1000"?
Click to expand...

"The Experts" are not measuring to an accuracy level of .001". These are just folks who have digital calipers which read to .001". If the caliper read to 0.000001", they would be writing figures to the sixth decimal. Or the tenth decimal place. And you could not convince them that they were not actually measuring something to that level, after all, they can read like anyone else. :banghead:
 
That is one reason some folks will be at the "wailing wall" with their dial calipers, because they refuse to believe the measurement. It seldom gets changed, but occasionally the scorer makes a big booboo and it will get fixed. :)

Losing by .0001 in an aggregate. Now that's..:banghead:

:D
 
+1 to the comments regarding the difference between precision and accuracy, and the difference between decimal places and significant figures. Suffice it to say that the differences are significant.

Also, regarding "measuring outside of one hole to the inside of the furthest hole and measuring from outside to outside and subtracting the caliber have equivalent accuracy," it is likely that the holes are NOT of caliber size, but are smaller. Experience this by measuring a hole without looking at the dial, then compare the dial reading with the bullet caliber. Therefore, unless you can PERFECTLY measure the "outside" of the hole at exactly the place that the outside of the bullet was as it pushed through the paper, error is introduced. So, I prefer the "outside-to-inside" method, since it will give pretty accurate and consistent results, as long as I consistently measure the "edge" of the holes, even if the "edge" isn't the actual edge.
 
program to measure group size

As stated by Walkalong measuring accuracy to .001 is as much an art as science. It's really hard to get an accurate group size because when a bullet passes through the target it doesn't punch a perfect hole.

Last year I was doing some ammo testing and I looked around for a program that would measure my groups. I couldn't find anything I really liked so I put one together myself. It calculates group size, average distance from center, and aim point to group center distance. It uses a picture of the target from a scanner or digital camera. I released it earlier this year as freeware.

Here's a link to my website where you can download a copy:http://www.ontargetshooting.com

Check it out a see if it works for you. The program isn't designed to replace existing methods of scoring competition targets but should give you enough information to compare groups for load testing.

Jeff59
 
How to measure accuracy to 3 decimal places?
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Nobody mentioned the subtract 1 MOA or add 100 yards rule. I believe that when measuring group size for the internet it goes something like this. Measure from the outside of the farthest bullet holes then subtract 1 MOA or add 100 yards. Some people do both. Makes for a lot better groups!!!!
 
I have seen some groups on the internet with pretty optimistic sizes posted with them. I would LOVE to measure some of them for the shooter. ;)
 
Oh yea, can you see the round holes in the plastic part of the Niel Jones group measurement tool? It has different caliber holes that are lined up with the "smaller than caliber holes" in the target. It is not really that hard to line up two circles.

Who is to say that the two measurements are EXACTLY as written. Tough call, but scorers have to make those calls all the time. (D*** 4 & 1 groups :banghead: :) ) These two targets are my way of reminding me about focus, or the lack thereof. What a huge difference if I get that 5th shot in. I don't even remember if it is #5 though. Could have been #2,#3, or #4 - All measures the same though

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