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  #1  
Old 09-06-2018, 01:22 PM
ERdept
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Measuring group size.. Do you measure center to center or outer daimeter?



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I see people post up measurements of groups size here.


And I see the calipers on the outer edges of the bullet holes in a group.


Are you guys using the above as the measure?

OR

Are you measuring above and subtracting half the bullet diameter to get center to center group size?

Which is the RIGHT way??
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  #2  
Old 09-06-2018, 01:50 PM
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I measure farthest outside to outside lead smear edges and subtract .224. It usually will give you exactly what on target program will give. That is for a 22 rimfire.
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Old 09-06-2018, 01:53 PM
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Case in point.


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  #4  
Old 09-06-2018, 02:05 PM
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Outside to out side, and then subtract the actual size of the bullet hole on your actual target. That will never be .224!
I use as a general rule of thumb .215. My holes shot on my 67# cover stock are somewhat larger than that at about .220, but I like to be safe and overstate my group size by just a bit.
On most paper including most factory printed targets, the holes will be even smaller.
More like .205 to .210. So you must measure an actual hole on your target. That my mean shooting a hole somewhere else on the target.

You can see how clean the holes are on that paper, and measure at .220.


(What I personally do is dial my calipers to .215 and then zero it. That way when I measure the outside edges the read out is the group size.
No need to do any math.)

Smooth

Last edited by Smoothtrigger; 09-06-2018 at 06:43 PM.
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Old 09-06-2018, 02:06 PM
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Depending on the paper used, it will stretch and the outer smudge marks on a hole may be substantially smaller than 0.224 inches. Typically, on card stock, my smudge marks are 0.204 to 0.211 inches. The best way to get group size is to shoot thee to five separate shots into the target paper at the bottom so you can measure and average the size of the holes. Then measure your groups for the largest outer marks for each group and subtract your average hole size to get a good CTC group size.
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Old 09-06-2018, 02:20 PM
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A lot depends on who is doing the measuring is what I have found.
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Old 09-06-2018, 02:41 PM
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The way I shoot, I just count how many of my hats it takes to cover all the holes.

Hector
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Old 09-06-2018, 05:07 PM
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At my indoor and outdoor range (22 rimfire only) five shot groups at 50 yards are all measured outside to outside. All measured from the outer most black smudge to opposing black smudge. Any group less than .400 is measured by three people and averaged. Then the rifle, ammo, and date are recorded on the target, along with the shooters signature, and the target is hung on the wall. The target will only be replaced when a better group is shot by the same shooter with the same rifle and same ammo. In seven years there are around 70 groups hanging on the walls, made by many shooters. There have been thousands of attempts to place a .400 or less on the walls. Most are made with target rifles and match ammo, only a few of those groups were made with a sporter rifle. The least expensive ammo ever used was Eley Target with a Savage MarkII BRJ, shot 4 years ago by my youngest son. The group measured .356 outside to outside. On a really good day you might shoot 2 or 3 groups, only the best one goes on the wall.
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Old 09-06-2018, 05:49 PM
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For unofficial stuff, I find a stand-alone or sighter hole and measure across it. It doesn’t matter whether I measure the actual hole or the lead ring mark as long as it consistent. Some 17hmr bullets leave tiny little holes and if they are round I’ll measure them. Then I measure across the longest direction of the group the same way and subtract the sighter size. That's center-to-center.

In OnTarget I do put the nominal bullet size in and center the mark on the hole each shot I enter. So that’s center-to-center measured differently.

Actual scoring is done using the method and equipment for the competition.

I have seen some talk about a group covered by a quarter / nickel / dime. Back in boy scouts I remember the rangemaster giving out a quarter, nickel, and dime award. He put the coin down and made sure no part of the hole showed. Maybe some people still do this.

The other piece of convention is group size casually discussed with no context. For Rimfire we usually mean 5 shot groups at 50y. For centerfire it’s 5 shot groups at 100y. Cherry picked group shot off a bench using the best ammo in ideal conditions. Unless noted otherwise.

Last edited by dgeesaman; 09-06-2018 at 06:00 PM.
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Old 09-06-2018, 07:25 PM
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OK, so this is the Factory test target of the Anschutz 54/30 I measure it with calipers as .432 edge to edge.


Is that right?

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Old 09-06-2018, 07:32 PM
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That's a little tricky because there isn't enough of a single hole to measure a hole diameter. But you should get a pretty close center-to-center measuring across the lead marks and subtracting 0.22.

I guess my question is how much does the test target matter? It looks pretty good but it was shot in a tunnel with a lot of ammo you don't have with the action bolted to a metal base.
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Old 09-06-2018, 07:46 PM
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Quote:
Originally Posted by ERdept View Post
OK, so this is the Factory test target of the Anschutz 54/30 I measure it with calipers as .432 edge to edge.


Is that right?

Your measurement is the raw group size, not the final, or center to center.

Here is a good explanation on measuring groups written by DrGunner, copied from one of his games.


MEASURING GROUPS, SIZERS & RAW vs FINAL GROUPS

Measuring Groups


The raw group size is the overall size of the group measured from outside edge to outside edge. What I have found to be the best and most consistent way to measure a raw group is to measure the outside edge of the bullet smudge on the paper from the widest point of the group using a caliper, placing the group deep within the flat part of the jaws of the caliper.
Sizers are used to calculate the final group size. What you're doing essentially is subtracting the size of one bullet with from the raw group size in order to end up with a final group size, Which reflects the center of POI for the two bullets that are farthest apart in the group. In order to calculate an average sizer, you shoot four separate sizers on a different area of the target from your groups. Measure the sizers in the same fashion as the groups – outside edge of the smudge outside edge of the smudge. Most sizers are around .210 – .215 inch.
Add up the four measured sizers and divide by four to get an average. Then you subtract that value from each of the raw group sizes to get a final group size for each of your groups. Just remember that the smudge represents the outer edge of the contact area of the actual bullet, and we are trying to calculate the group size from point of impact to point of impact. Subtracting a sizer results in removing 50% of the average width of a bullet from each side of the widest part of the groups, giving you the center of POI for the widest part of the group.


Because ammo and paper varies in the marks left on paper, doing it this way ensures that we are taking into account the way that your particular combination of rifle/ammo/paper is factored in by using your sizers. Because we are measuring the smudge, shooting a few fouling shots in order to dirty the bore helps a lot because subsequent bullets will print a better/more distinct mark on paper that is easier to read and see.

One good practice to help you understand why we are measuring the outside edge of the smudge is to plug a target – In particular, a sizer. Take a live unfired round and push it slowly into a sizer hole up to the base of the bullet just before it contacts the casing. You will see that the width of the hole made by the bullet is actually larger than you would expect based on the hole in the paper. The smudge left behind by the passing bullet provides a reasonably accurate estimation of the actual size of the hole, especially when averaged across four separate sizers

Last edited by Hawkeye57; 09-06-2018 at 07:48 PM.
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  #13  
Old 09-06-2018, 08:03 PM
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Quote:
Originally Posted by ILIKE1022 View Post
I measure farthest outside to outside lead smear edges and subtract .224. It usually will give you exactly what on target program will give. That is for a 22 rimfire.

.224 is for centerfire and .223 is for rimfire. I also measure outside to outside and subtract .223 from that measurement for rimfire only. Any other way is not correct.
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Old 09-06-2018, 08:08 PM
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Quote:
Originally Posted by dgeesaman View Post
That's a little tricky because there isn't enough of a single hole to measure a hole diameter. But you should get a pretty close center-to-center measuring across the lead marks and subtracting 0.22.

I guess my question is how much does the test target matter? It looks pretty good but it was shot in a tunnel with a lot of ammo you don't have with the action bolted to a metal base.

Using your method the group is then .212?

does it matter to me, ...only as a benchmark.

Im going to use my own means to try to gain more accuracy.

Last edited by ERdept; 09-06-2018 at 08:12 PM.
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Old 09-06-2018, 08:19 PM
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Quote:
Originally Posted by ERdept View Post
Are you measuring above and subtracting half the bullet diameter to get center to center group size?
Not half.... the whole. That's the same as subtracting half from each side to get to the center.
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