Bullet grouping measurement?

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Oden

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GunNutz
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Reading an article in G&A this morning about accurate rifles. Writer claimed to have a 1/10 of an inch grouping with rifle "x".
Got me to thinking "What is the correct way to measure a grouping?"
I have always measured from the outside of the two widest spaced holes to get my measurement.
Have I been cheating myself out of a few tenths of an inch all these years?
Cheers.
 
You measure from center of hole to center of hole. And for grouping, you use the two holes the furthest apart.

In official competion, especially handguns, you can use a tool you insert in the hole or a plastic template to determine the center of the hole.

Hope this helps,

Duke1
 
To get center to center is tough in some instances....BR for example where you have one hole.

Fire a gage hole (reference for the paper media you are using) and measure its diameter.

Measure the one hole group outside to outside at the widest point and deduct the "gage" hole dimension.

Most other groups (distinct holes) can be measured radius (or edge) to radius at the widest point using the same side of each hole. Lets say one bullet at 12 and one at 6. Measuring bottom hole edge to bottom hole edge will yield center to center group size.
 
Why do you subract the diameter of the gage hole for bench rest, but not for the other group you mentioned? Or did I miss something
 
what i do is i use an electronic caliper, measure the outside of the 2 furthest hole, and deduct bullet diameter, in my case, .308. so i might get groups that are say .859in in diameter, substracted by .308 = .551in group, center to center
 
Well you can measure outside to outside and subtract the gage hole if you like, but that involves math.


The problem with deducting bullet diameter is that often times the hole is actually smaller then the bullet. So when you deduct the actual bullet dia. you get a smaller group...:eek:
 
Or you get an electronic caliper and zero it at your bullet diameter, (i.e., open it to .243 and then zero it) then just measure outside edge to outside edge, no math involved this way!
 
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Keith - keep in mind your reference diameter will likely be different from the paper hole, giving you a smaller group size...
 
if there are distinct holes you can go from far edge to close edge, giving you center to center a different way, doesnt work so well when holes overlap though.
 
TheCanuck said:
what i do is i use an electronic caliper, measure the outside of the 2 furthest hole, and deduct bullet diameter, in my case, .308. so i might get groups that are say .859in in diameter, substracted by .308 = .551in group, center to center
Click to expand...

X2

If your holes on paper are a tad smaller then the true diameter, then use the hole diameter in your math instead of the true caliber. Either way, it's not good enough to measure a hole in a piece of paper with calipers and expect .001" accuracy.

If you are really fussy, place your shot target in front of a profile projector;)
and you can get a very accurate center to center measurment to the .0001". The thing being is that you must have a projector:D
 
:eek: More equipment..:D but it doesn't sound like fun equipment or high value equipment:eek: Calipers work just fine for general target work.
 
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