Why you're almost certainly measuring your groups inaccurately.

flashman

flashman

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It seems the most common, and most accepted, method for determining group size is to measure outside edge to outside edge, and subtract one bullet diameter, as is the specified standard for the MOA challenge threads above.

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1.007 - .308 = .699 = .67 MOA

But that's not really the group size. This method has one serious flaw; it relies on an assumption that the hole in the paper is bullet diameter. It almost certainly isn't. It also forces us to do math, when math isn't necessary at all.

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Uh oh. .265" That's optimistic by .09" if we measure outside edge to outside edge.

The accurate way is to measure outside edge to inside edge; regardless of hole diameter, that will give us the true center to center, with the added bonus of there being no need to do arithmetic.

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Not to open a can of worms, but groups measured outside to outside with a bullet diameter subtracted are almost certainly going to be inaccurate, in favour of a smaller group size than is reality. It always seemed odd to me that the most commonly quoted method is also the most inaccurate one; especially if we're talking precision rifle. Especially if we're talking scoring precision rifle.
 
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your method only works if you can measure exactly perpendicular to the farthest point to the nearest point of the two holes, this perpendicular line has to be drawn exactly through the center of both holes, almost impossible to do with nothing more then hand tools and naked eyes.............
 
Stormchaser said:
Isn't the flaw measuring 3 shots and not 5?

cou:
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LOL!

Too funny. =)

I'm cheap, it's a load workup, what can I say. Pushed just over a hundred rounds that afternoon out of my Sako Black Bear in .30-06; keen to fire more I was not.

Not a precision rifle I know, it was the first target to hand when I reached over to the shelf just now. Point holds true however, no matter the caliber or rifle.

Is it a huge deal? Goodness no, it's minor. But this way is easier, and has the advantage of being more accurate.
 
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Instead of measuring an irregular-sized hole all the time; can you not center your calipers, in the same way you center the front post through a peep sight. Is this really any more or less accurate, than measuring arbitrary, irregular points?
 
It doesn't really matter what we do as individuals... any group competition I have attended had one person doing the measuring with a proper measuring tool. If any of those groups were possible records they were sent to another person for official measurement.
 
yodave said:
your method only works if you can measure exactly perpendicular to the farthest point to the nearest point of the two holes, this perpendicular line has to be drawn exactly through the center of both holes, almost impossible to do with nothing more then hand tools and naked eyes.............
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I'm not sure I follow Dave - you're measuring exactly the same way as if it were outside to outside. Outside to outside also inherently introduces a significant error, which inside to outside does not.
 
guntech said:
It doesn't really matter what we do as individuals... any group competition I have attended had one person doing the measuring with a proper measuring tool.
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I was trying to find a link to the magnifier with the multiple diameter circles on it that we use here on the island, but then I thought about how important it was and moved onto plucking that 1 annoying hair that sticks out of my eye brow
 
CoonT said:
Instead of measuring an irregular-sized hole all the time; can you not center your calipers, in the same way you center the front post through a peep sight. Is this really any more or less accurate, than measuring arbitrary, irregular points?
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I think that would be just as accurate too; certainly more accurate than outside to outside minus bullet diameter. The human eye is remarkably good at resolving small tolerances; down to about .0004"
 
flashman said:
I'm not sure I follow Dave - you're measuring exactly the same way as if it were outside to outside. Outside to outside also inherently introduces a significant error, which inside to outside does not.
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outside to outside is easy to define, finding the edge of a circle to measure off of to a point on another circle is not, you can easily be off, the other thing to consider is that very few holes in a target are ever perfectly round, most holes are in fact ovals.........
 
Theoretically that is correct flashman, and you do highlight a valid technical point. But I think on a more practical side, and as stated by yodave, measuring with hand tools and eyes most certainly will not give you a precise measurement to the .001". If this is what you are in fact looking for as far as precision measuring goes, then you must also take in consideration that, the same way each hole diameter isn't the nominal bullet size, the same way you can't guarantee the hole centers. Add to that the inconsistency in readings, I think sportsmanship prevails in this case, and is satisfactory for most.... Just my 2 cents ^_^
 
yodave said:
outside to outside is easy to define, finding the edge of a circle to measure off of to a point on another circle is not, you can easily be off, the other thing to consider is that very few holes in a target are ever perfectly round, most holes are in fact ovals.........
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I like that too - doing it your way makes it .710, not .725. I find inside to outside pretty easy to define well, certainly either way is much more accurate than outside to outside minus. Measure a hole, zero out and go outside to outside is much more accurate also. Certainly, there seems to be no disagreement that outside to outside minus bullet diameter introduces some blue sky to group size, and has the additional complication of creating an arithmetic problem.
 
Tools like this don't cost that much if you are really concerned. Neil Jone parts, a Hornady caliper, a magnifier and some plastic...

The reticle is a small hole with concentric circles around it. It took a bit pf practice to scribe the circles...

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guntech said:
Tools like this don't cost that much if you are really concerned. Neil Jone parts, a Hornady caliper, a magnifier and some plastic...

The reticle is a small hole with concentric circles around it. It took a bit pf practice to scribe the circles...
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That's WAY anal retentive.

My preferred flavour is mildly anal retentive.

:d
 
if you can't measure your group then go back to shooting for score lol, as you get better us a smaller target..............
 
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