Measuring group sizes

Epoxy7

Epoxy7

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Well I just bought an electronic caliper "with digital display" from Crappy tire (It's on sale $15). I thought this would work well for checking those group sizes. If nothing else it'll stop me from claiming "internet" group sizes :D.

So I gather there are two ways to measure. From centre to centre (which I don't like) and from edge to edge. Only with the edge to edge you remove the diameter of the bullet?

So here goes. How do I calculate this group?

5 rounds shot at 100 yards with a .308.

Caliber measurement from edge to edge = .654 inches. This was taken from the longest distance. IE the horizontal was longer than the verticle distance.

So was this a .654 inch group or do you remove the diameter of the bullet? IE .308?

Thanks
 
You subtract the diameter .308 to get the group size.

If you shot 3 shots through a single hole, the group size ought to be zero.
 
If you subtract the diameter of the bullet from the outside measurement, then you will have the center to center measurement, e.g. .654-.308= .346. So, you have a .346 inch group size. I'm no expert on measuring group sizes, but for the longest time I have taken the maximum distance between the bullets in the group, like you have, and subtracted the caliber size because I was interested in where the center of the bullets hit rather than the outside edges. The other thing I do is flatten the paper when measuring the group and measure from the edge of the black mark on the paper rather than the hole made by the bullet. I feel that this is probably closer to where the edge of the bullet actually was. (Most of the time I am using cheap paper that tears.) In fact, I have shot the odd (Note the word 'odd'.) group at short range (When I have been using my Savage, of course. :D) that looked too close to the caliber size unless I did this. I'm not saying my way is the right way, or necessarily the best way, but if a guy is consistent, at least he can make valid comparisons between his own loads and clearly tell others how he got them. There are, of course, other things a guy can look at besides maximum spread. One thing I do that I'm sure other guys do, is keep an eye on vertical as opposed to horizontal spread. It will be interesting to see if anyone else has a different approach. Cheers! Nice group, by the way. Guess you'll have to get used to internet groups. :) Fred
 
This has been said so many times... :)

Groups are measured center to center of the two holes farthest apart...

...everyone agrees on that... BUT you don't always subtract the caliber...

If you don't have a tool that measures to the center of holes, then it is acceptable to measure to the outside of the holes or to the outside of the "mark"... and then subtract... BUT you do not subtract the caliber unless the caliber happens to measure the same as a single hole on that paper... OFTEN the hole or mark made by a bullet on a target is different than the caliber... very slightly but different..

I would say it has very little effect on any groups measured by the majority of gunnutz members.. it is just a point that needed making...

When registered groups are being measured sometimes a single hole on the target is viewed by the person measuring to sort of calibrate in their mind how to hold the tool on the group... in registered groups a few thousands on an inch can easily decide a match.

Those who have done this will recognize what I am saying...
 
ben hunchak said:
Measure from centre to centre of the furthest apart holes....for a Gunnutz group; subtract 2".
Click to expand...


Hahhaha. That was kind of funny.

Some good responses.
I guess it's correct that the hole size may very well be different than the diamter of the bullet. That being said, a person's guestimate of the center of the hole may also be slightly off, especially when you have multiple rounds in the one hole.

Using the edge (black mark) to edge and subtructing .308 as noted in the above posts gives me a .346, for a 5 round group at 100 yards with 168 grain Federal Gold Medal ammo. Not bad for the first 5 round group with my Remington 5R.

I'll try remeasuring from Centre to Centre as has been suggested to check the difference.
When I get a second I'll also check to see the actual size of the .308 single hole.

Thanks for the responses everyone.
 
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The way you measured it is a close enough result to tell you how your rifle is shooting. I would assume that 99.9% of the people here who measure groups do so subtracting the caliber and not the printed hole size.
 
When you measure to the center of a hole you use a device made to center on it...

Here are two pictures of one such tool...
the top view
measureing%20device%201.JPG


From the bottom
measureing%20device%202.JPG


Here is what it looks like sighting on a hole...
measuring%20device.jpg


As you can see, the hole is slightly different than the smudge mark... the smudge mark is used for centering... it is truer than the torn hole... that is a .22 LR caliber hole and it only measures about .125".... the smudge mark measures about .215".
 
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Guntech thanks for the info.
I naturally don't have one of those. Oh well I'll get there eventually.

By the way I have one of the rifles you worked on. Very nice. I haven't had a chance to shoot it yet but it's a neat wildcat. 7mm WSSM. I bought it from Ardent.
 
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