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Old October 19, 2014, 01:28 PM   #1
dnttech
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Join Date: December 7, 2011
Location: Michigan
Posts: 3
Measuring Sub-Caliber Group Size

Hi...As a new member here, I have a question for those who may be able to help. I recently shot a 100 yard, 3-round group from the bench where the resulting hole measured smaller then the actual diameter of the bullet.

In this case, exactly how is the group measured? Is it simply a matter of measuring the hole edge to edge? If this is the case, the hole measured .288 while the bullet measured .308. Would this then be measured as a .288 inch group? I obviously can't subtract the diameter of the bullet since this would give me a negative number.

While I've been able to shoot quite a number of groups in the .00x range, this is actually my first time shooting a group smaller then the diameter of the bullet.

I've asked quite a number of my fellow shooters how to approach this, and quite simply, no one was sure.

Thank you in advance for any help you may provide.

P.S. By the way, for those who may wonder why this isn't a 5-round group, I just didn't load up enough rounds. This was a test load I was working up for this particular rifle.

P.S.S. I've tried loading a picture of the actual target, but apparently the size of the image is too great.
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Old October 19, 2014, 01:51 PM   #2
Ske1etor
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Join Date: April 17, 2008
Location: Marrero, Louisiana
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http://www.ontargetshooting.com/download1.html

This is how.

With a ruler or caliper, measure from outside of grease ring to outside of grease ring and subtract bullet diameter. If your single hole is inside the bullet diameter, the spread is 0.
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Old October 19, 2014, 02:04 PM   #3
JohnKSa
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Join Date: February 12, 2001
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Quote:
...the resulting hole measured smaller then the actual diameter of the bullet.
It's not unusual for a bullet hole in a paper target to be smaller than the bullet diameter itself--sometimes it is significantly smaller. This is why I take every opportunity to decry the method of measuring edge-to-edge of the outside shots and subtracting the bullet diameter. That method almost always underestimates the group size.

As your group proves, it's even possible to shoot a group and end up with the overall size of the hole in the target being less than the bullet diameter. Your overall group size is more than 6% smaller than the diameter of the bullet you were using.

One option would be to always shoot a few test shots outside the group to characterize the size of the hole that the bullets make in the paper. Carefully measure the holes, average their sizes and use that average number to subtract from the edge-to-edge group size rather than simply using the nominal bullet diameter.
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Old October 19, 2014, 05:14 PM   #4
dnttech
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Join Date: December 7, 2011
Location: Michigan
Posts: 3
Measuring Sub-Caliber Group Size

Ske1etor and JohnKSa, thank you both for your input.

Ske1etor, I used exactly your method for determining the hole size...outside of grease ring to outside of grease ring, with a caliper, and came up with the .288 measurement.

While my normal bench rifle is a 6br, I'm happy I was able to work up a load with which the rifle was happy. I'm just getting my son into bench shooting, and the .308 he's using with the round I worked up should suffice for the time being.

Again, my thanks...

Bob

P.S. JohnKSa, my apology for neglecting to mention your input. Your suggestion of shooting several shots (I'll do 5 shots) and determining the average of the shots makes a lot of sense. For the number of years I've been shooting, I can't believe I didn't think of that. Thank you....

Last edited by dnttech; October 19, 2014 at 07:32 PM. Reason: Forgot To Mention JohnKSa
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