Measuring group size with a caliper

[LiveLife]

Well, I used to use a ruler and tried to "estimate" the center-to-center of the holes, but this video uses the caliper to measure the outside-to-outside of the holes and subtract the diameter of the bullet to obtain the group size (more accurate).

You learn something new everyday.

http://www.youtube.com/watch?v=_vuw9iHzgIY&feature=related

 

[Walkalong]

Draw a line between the two bullet holes furthest apart going through their center. Measure the outer edges at the line. If it is a tight group you may need to draw more than one line and measure both to see which is largest.

 

[LiveLife]

Hmmm, now I learned two things today. Thanks!

 

[Walkalong]

Like this. Yep, that was a good one. .144 @ 100 yds

 

[LiveLife]

Now, that's a small group.

 

[Jesse Heywood]

So are we know checking to see who's is smallest? If I shot a group that small I would have to see if I still had a pulse.

Easiest way to check group size is with a circle template. I would draw them on the computer and print on paper. Then go to the copier with view-graph material.

 

[TexasShooter59]

I just started using a software program called OnTarget found here. It seems to work pretty well. It does require you to take a photograph of your target (or scan it) and load it onto your computer.

A tip: set your reference before adding the shot circles so the circles will be the correct diameter.

 

[Offfhand]

Special instruments are made for precisely measuring group size. Attached is a photo of one made by Neil Jones. It works by centering the appropriate size ring over one of the widest holes, then extending the instrument so the same circle surrounds the opposite widest shot and reading spread on caliper. The attached magnifying lens helps to more precisely center the rings. Instruments of this type are used in Benchrest matches when exact measuring and scoring is required and they require some practice to use with necessary accuracy. Obviously, such exact accuracy is unnecessary for casual measuring and not worth the time and effort.
A very fast and sufficiently accurate way to measure groups is with digital calipers. Simply open the caliper to the appropriate bullet diameter (.308" for .30 cal. etc.) and press the zero button. With Caliper closed it then shows a negative number of your bullet diameter. Thus when you measure the group from outside to outside it automatically subtracts the bullet diameter and you read the actual group size. Simple

 

[R.W.Dale]

If your group is made up of separate holes just measure from the inside edge of one hole in relation to the group center to the outside edge of the other extreme bullet hole.

Done no math required.

 

[twofifty]

For match rifle bullets (not SWC or WC) do you include in group size what I call the 'grease ring' (which is likely smeared barrel fouling)?

When scoring for points at informal matches, I figure that if the 'grease ring' touches a target ring, the higher score is attributed.

 

[LiveLife]

Well, I can tell you I learned a lot about measuring shot groups this week.

When scoring for points at informal matches, I figure that if the 'grease ring' touches a target ring, the higher score is attributed.

Some pistol match scorers still will count only the edge of the cut hole, not the grease ring and some will. And you can't argue the scoring decisions on some matches. If I am shooting matches, I will use SWC when I can over RN for better scores.

 

[TH3180]

I use on target, it does all the work for you and it's free.
http://www.ontargetshooting.com/

 

[LiveLife]

TH3180, thanks but the program won't run for those of us using Linux/OS X.

I think I will blow the dust off my Windows XP and install it on another drive.

 

[ArtP]

I'm another one who uses on-target software. It's fantasitic and well worth loading up an XP partition. Quite effortlessly, it is accurate to a thousandths of an inch. It has some other features you may find interesting as well.

 

[Otto]

Yeah, it's a good program that lets you save your targets to a hard drive.
My son shot this group 2 weeks ago with his new rifle. I gave him a box of Federal .308 to practice on and this is the target he printed after sighting-in.
This was the first time that he ever fired a centerfire rifle.