How To Properly Measure Groups

trky chsr

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What is the proper way to measure group size? I used to measure center to center but I read somewhere you should measure outside spread and subtract the bullet diameter. This seems to work for me except for the following example:

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This is the best group I've ever shot. This is a 5 shot group at 50 yds. CZ 452 Varmint (stock) Eley Standard (Yellow) off bags. I shot 4 5 shot groups and the other 3 were all around 3/8". I have an At-One as well and it's a very fine shooting rifle as well. TC
 

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I'd say from the center of the bullseye, to the center of the group.

Tight group matters not if you're off target, when trying to bring home the bacon.:rolleyes:
 
What is the proper way to measure group size? I used to measure center to center but I read somewhere you should measure outside spread and subtract the bullet diameter.

The measurement is the same as you have stated. In other words:
Look for the longest distance outer edge to outer edge. Then,
Measure from the edge of the outer edge of the bullet carbon smears. This is where the bullet touched and cut through the paper for sure. The bullet will pull petals of paper into the hole with it. The carbon smear on the petals indicates where the bullet touched the edge of the true hole. Sometimes you need to push the petals back towards the entry direction to see the distinction between white (or orange in your case) paper and black or gray carbon smear.

Then subtract the bullet diameter. That is your group size. Same process for a single group hole, or individual holes in a group. A group is a group.

Measurement is best done with calipers.


The outer edge of the hole or torn edge is not necessarily where the bullet touched the paper. The paper can be torn in many ways beyond where the bullet cut through. e.g. sometimes debris underneath the paper on the target backer will fire back through the hole and tear more of the paper away.

Sometimes the smear and the edge of the cut hole are almost identical. Press the petals or curl back inwards from underneath and you should see a distinct demarcation between white (orange) paper and the black or gray carbon smear.

If the target paper is jet black, (not recommended for shooting groups obviously), the carbon smear should contrast as gray-ish.
 
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Biologist has the answer. 'Measure the outside smears' and subtract. And heavier paper like posterboard will give easier measurements than 'copy paper'. I print my targets on copy paper then glue them to posterboard (or box-board like cereal boxes) Just an aside note, the 'bullet diameter' should be from a bullet 'slugged' thru the barrel, not the nominal bullet size. The 'groove of the rifling may be a few thou different than 0.22", depending on the rifle make and model. Some brands have diff grooves in diff models, like 'target', hunter, sporter, etc. And some manyfacturers will give you an 'actual' bore size in the specs.
 
Measuring group size has nothing to do with a bullseye or measuring the bore.

It is more accurate to measure the outside spread and subtract what a single bullet hole measures, not the bullet diameter. Often a single bullet hole measures differently than bullet diameter...

In the early stages of organized Center Fire Benchrest Shooting in the late 60's we always made sure there was a single bullet hole on the sighter to measure so the scoring group could be accurately measured.
 
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Measuring group size has nothing to do with a bullseye or measuring the bore.

It is more accurate to measure the outside spread and subtract what a single bullet hole measures, not the bullet diameter. Often a single bullet hole measures differently than bullet diameter...

In the early stages of organized Center Fire Benchrest Shooting in the late 60's we always made sure there was a single bullet hole on the sighter to measure so the scoring group could be accurately measured.

this!
 
This target was acquired from Rimfire Central. One shot was fired at each small target and five shots were fired at each large target.
Each single shot was measured to determine the projectile size. The four groups were measured and the average measurement was selected to get the average group size.
Using cover stock or index card for your targets gives a nice clean hole that is easy to measure. Using 20 pound paper produces holes that are almost impossible to measure.
Shoot'n'Score targets do not produce impacts that are easy to measure.
In the 5-shot 50 yard challenge, a picture of the largest group with the measuring device in place is an example of the measuring procedure.
This target was shot at 100 yards. The three group average was not used but someone may find an application for it.
Group size is important when testing ammo . . . scores are saved for game day!

 
Measuring group size has nothing to do with a bullseye or measuring the bore.

It is more accurate to measure the outside spread and subtract what a single bullet hole measures, not the bullet diameter. Often a single bullet hole measures differently than bullet diameter...

In the early stages of organized Center Fire Benchrest Shooting in the late 60's we always made sure there was a single bullet hole on the sighter to measure so the scoring group could be accurately measured.

This is not the way it works in benchrest today. Exterior spread less bullet diameter used.
And for the OP : calliper not a ruler.
 
This target was acquired from Rimfire Central. One shot was fired at each small target and five shots were fired at each large target.
Each single shot was measured to determine the projectile size. The four groups were measured and the average measurement was selected to get the average group size.
Using cover stock or index card for your targets gives a nice clean hole that is easy to measure. Using 20 pound paper produces holes that are almost impossible to measure.
Shoot'n'Score targets do not produce impacts that are easy to measure.
In the 5-shot 50 yard challenge, a picture of the largest group with the measuring device in place is an example of the measuring procedure.
This target was shot at 100 yards. The three group average was not used but someone may find an application for it.
Group size is important when testing ammo . . . scores are saved for game day!


Group size is used in benchrest rimfire along with score. One is not going without the other. In score you have to make sure each bullet in your group cover the x. A lucky group won’t cut it. It have to be consistent.

The best is to practice with benchrest target for group and score. No red dot, no grid..just plain target as used in matches. The benefit is you can calculate after each target, your score, and find out how you do with that ammo- system.

Rimfire target..scope.
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Thanks for all the replies, they are very helpful. treboor2880 I read a long time ago that there is accuracy and precision. Accuracy is hitting where you aim and precision is shooting very small groups. Some rifles are very precise but not accurate and some rifles are very accurate but not very precise. Of course a really good rifle is both! Janeau, ruler is for the photo only, I do measure with calipers. TC
 
This is not the way it works in benchrest today. Exterior spread less bullet diameter used.

No. In rimfire benchrest competition no one shoots groups. RFBR competitors shoot for score. But when they do measure group size they understand that the bullet's hole in the paper will be smaller than its diameter and take that into account. Using the bullet diameter gives a group size that's smaller than it really is.

Of course, for measurements of a more casual nature, many shooters accept using the bullet diameter.
 
Maybe I'm just dense, but how can the hole be smaller than the diameter ? Do you mean the 'diameter as on the box' or a measured unfired bullet? If you mean that there is a possibility of 'tears' that leave a smaller hole I can 'see' that but I've been under the impression that in those cases one would use the 'smear' of lead as the definitive measurement. I've 'slugged' a couple of my rifles and have seen the 'actual' (with a $25 digi-caliper) flat & groove sizes so I don't believe in 0.22" for bullet size but as you said above, I use diameter for my calcs as I have not slugged all my rifles and am not in any formal competition. A couple thou don't matter at my level.
 
What is the proper way to measure group size? I used to measure center to center but I read somewhere you should measure outside spread and subtract the bullet diameter. This seems to work for me except for the following example:

View attachment 560819

This is the best group I've ever shot. This is a 5 shot group at 50 yds. CZ 452 Varmint (stock) Eley Standard (Yellow) off bags. I shot 4 5 shot groups and the other 3 were all around 3/8". I have an At-One as well and it's a very fine shooting rifle as well. TC

Try not to over complicate the procedure. Definitely use digital caliper and go with Biologist's post OR download Ballistic-X app to measure groups and save the pictures to your phone for future reference.
 
Using 20 lb paper you will get tears.
Using Index card or Cover stock in the 65 lb range, when the bullet passes through there is shrinkage and the paper retracts from the lead smear.
I have measured the lead smear down as low as 0.189.
To my amazement, when a group in measured as 0.723 for the 1/2" challenge, then subtracts 0.224 for a group size of 0.499".
Using quality 65 lb paper, it is impossible to get a true measurement larger than 0.22 . . . IME!
 
Maybe I'm just dense, but how can the hole be smaller than the diameter ? Do you mean the 'diameter as on the box' or a measured unfired bullet? If you mean that there is a possibility of 'tears' that leave a smaller hole I can 'see' that but I've been under the impression that in those cases one would use the 'smear' of lead as the definitive measurement. I've 'slugged' a couple of my rifles and have seen the 'actual' (with a $25 digi-caliper) flat & groove sizes so I don't believe in 0.22" for bullet size but as you said above, I use diameter for my calcs as I have not slugged all my rifles and am not in any formal competition. A couple thou don't matter at my level.

The "hole smaller than the diameter" means that the paper towards the center of the bullet's impact, i.e. the nose, blows out the center of the hole, but the outer metal of the bullet does not. Instead the the outer metal of the bullet bends the paper (around the nose's hole), inwards until it tears into several petals and lets the bullet pass through. The tearing of the petals can radiate quite far away from the hole. When you reach behind the paper and push the petals back you can see the lead/carbon smear ring on the inner ends of the petals, which indicates exactly where the bullet touched the paper. The outer edge of the smear is the outer edge of the group, whereas the actual hole with the petals bent back inwards is much smaller than the smear radius.

In rimfire, that outer smear line determines score for the round. At my range the scoring rule uses inside edge of the smear. If the smear cuts the line of the inner ring, then it scores for that ring. Its not the hole, its the smear. But that said, we also use an Anschutz .22LR "official" scoring plug with a magnifying lens on it (purchased from Nordic Marksman). When the plug is inserted in the hole, the petals will be bent back since the hole is usually smaller than the plug. In the magnifying glass you can see if the plug "cuts" the ring line. Even though the petal tears usually radiate out well past the diameter of a bullet, the plug tends to self-center itself because of the friction of the springy-ness of the paper and petals pushing it back into the center of the "hole".

We use "card stock" (which I think is 65lb stock) for printing targets. You need this minimum thickness and density of paper for scoring plugs to work consistently true. Regular writing paper tears too much in irregular holes, and the hole or weakness in the petal's springy-ness will not allow a true consistency for a scoring plug.

In centerfire score our club uses some home made plugs (several calibers), using the same inside edge rule for scoring. Nordic Marksman also sells scoring plugs for various centerfire calibers.
 
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Maybe I'm just dense, but how can the hole be smaller than the diameter ?

If a .22LR bullet was shot through something like aluminum foil, the hole would probably be very much the same as the bullet's diameter. Paper responds differently and as a result a .22LR hole will be smaller. How much smaller will depend on the paper used.

Below are two images showing .22LR bullet holes in cardstock, either 65 or 80 lb weight (I don't remember which).

The first shows a caliper set at .220" and the hole, including the grey smudge, is obviously smaller in diameter. The second shows the actual diameter of the .22LR bullet hole, which in this paper measures .205".



 
Maybe I'm just dense, but how can the hole be smaller than the diameter ? Do you mean the 'diameter as on the box' or a measured unfired bullet? If you mean that there is a possibility of 'tears' that leave a smaller hole I can 'see' that but I've been under the impression that in those cases one would use the 'smear' of lead as the definitive measurement. I've 'slugged' a couple of my rifles and have seen the 'actual' (with a $25 digi-caliper) flat & groove sizes so I don't believe in 0.22" for bullet size but as you said above, I use diameter for my calcs as I have not slugged all my rifles and am not in any formal competition. A couple thou don't matter at my level.

It varies on what paper is being used on how large a pointed bullet cuts a hole. They never cut bullet diameter.
 
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