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Thread: Group size questions

  1. #1
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    Default Group size questions

    How do you guys measure your bullet groups? I have always compared the shots to some known measurement. IE: Three shots under a quarter, or three shots in a 2" circle etc... What is the "official" way, and how does everybody else do it. Thanks, Charlie
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  2. #2
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    Center of the bullet hole to center of the bullet hole of the two that are farthest apart.

  3. #3
    Member JOAT's Avatar
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    Basically the same, but throw out any obvious "fliers" first. Since the center of a hole is not easy to find, take a straight line from the outer edge of the first hole to the inner edge of the second one. This gives you the same as a center to center measurement.
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  4. #4
    New member George's Avatar
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    Default center to center

    This shows an easy way to approximate the group size.

    outside edge of farthest two____________ .960"
    minus bullet dia. (270 shot this group) ___- .277"
    equals group size_____________________= .683"

    for official scoring .277 plugs would be inserted in the two holes
    and the measurement taken from their outside edges.
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    Member JOAT's Avatar
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    I think it would be easier if you run the fixed end of your caliper in to the inner edge of that left bullet hole. Then you measure the distance directly off the dial without any math. It is, as you said, an approximation, so there is no need for 3 decimal places of accuracy based on bullet diameter.
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  6. #6
    New member George's Avatar
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    Default group size measurement

    mnbirddog,
    Regardless of method, it is center to center between the farthest holes. For practical purposes I guess you can just eyeball the farthest centers with a ruler to the nearest 1/8" and call it good. The problems start with really accurate rifles where they can shoot out one ragged hole. Then the question becomes 'where is a bullet hole center to measure from?'. Using a caliper from outside to outside then subtracting the bullet diameter will give the desired center to center group size measurement. Even using bullet caliber scoring plugs then measuring would barely be "official" enough for most club and some regional competitions. These are done at the .001" increment. But, for sanctioned, larger shoots the current accepted method employs a dedicated caliper mounted to rigid, flat clear plastic plates that have inscribed, bullet sized rings at each caliper end. Each jaw end center is at the center of each ring. The scoring tool is placed over the target and the farthest outside holes are centered under the rings inscribed at each end of the calipers. A direct center to center reading is then taken. The target is then turned over and scored again. The largest of the two is then recorded as official. At the current level of accuracy in many of the benchrest classes the 001" level of discrimination is only barely adequate to prevent most ties or contested results. So I guess there is "official" and then there is "official".

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