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Group measurement

Discussion in 'Competition Shooting' started by TheRodDoc, Mar 3, 2010.

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  1. TheRodDoc

    TheRodDoc Member

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    I have never shot at a range. I have wondered how they measure the small groups like .200" in benchrest shooting.

    How are the targets set up so the holes are clean enough to measure that accurate? And what do they measure with?

    I have a 220 swift that shoots groups that measure very roughly 5/16" to 3/8" at 100 yrds. but hard to tell exactly for the holes aren't clean. This is by drawing a circle around the outside of all the holes as close as I can tell where the outer edge of each hole is.

    Do they measure by drawing a circle around all the holes? Or bullet hole center to center.
     
  2. fineredmist

    fineredmist Member

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    Group size is measured center to center. Benchrest shooters use special instruments to measure the groups. May I suggest that you do a search for group measuring under benchrest to get all the details. It is a "simple" measurement process that gets really complex in a hurry.
    If you convert the inches to thousands of a inch and subtract .224 you will get the "real" group size.
     
  3. twofifty

    twofifty Member

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    The above is not quite correct, as group sizes are stated to be X thousandth inch center to center.
    Groups are usually measured outside to outside of the holes that are furthest apart, then by subtracting the bullet diameter.
     
  4. Walkalong

    Walkalong Moderator Staff Member

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    Yep - Example.
     
  5. TheRodDoc

    TheRodDoc Member

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    Ok. I understand the how now. but I still don't see how the hole edges are accurate enough to get a measurement of lets say .353 . Couldn't it have just as easily been .350 or .357 or .359. A thousands really can't be seen by eye. Even 5 to 10 thousands would be hard to see.
    Even using a caliper it is still up to the eye to judge where the tips are to the hole edges.
    plus the targets I shoot don't have very clean hole edges to measure to the thousands.

    I would try to measure mine that way if I knew how.
     
  6. Jim Watson

    Jim Watson Member

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    Not my game, but I have seen pictures of benchrest scorekeepers with modified calipers. They have transparent plates etched with a reticle. I have to assume they are trained to locate the edge of the "one ragged hole" groups.

    The world record was range scored at .000" literally five in onehole; but review at headquarters concluded it was really .009"
     
  7. Walkalong

    Walkalong Moderator Staff Member

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  8. twofifty

    twofifty Member

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    Wow, that is one slick unit.

    RodDoc - for everyday practical applications, a trained eye will be able to discern from the bullet smudge where the edge of the hole is. Do not count the long radial tears, but you can usually include the fuzzy closely-packed paper fibers, these are often part of the hole. You can verify if that is the case by pushing an identical bullet through the hole - the bullet will center itself in the hole, it will push the fuzzies to the side, and you will see that it likely covers the smudge marks.

    With a bit of practice you will 'train' your eyes, and so long as you are consistent, you can measure your progress.
     
  9. TheRodDoc

    TheRodDoc Member

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    Thanks for all the answers.

    I have never had till now a gun that would shoot small groups where the holes over lap each other and are harder to measure. With any of my other rifles the groups are large enough to have the holes completly apart from one another making it easier and to me not so critical to measure.
     
  10. twofifty

    twofifty Member

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    Rifles in 220 Swift are unusual now. Can you tell us about this rifle: action-barrel-trigger-stock/bedding-optics?
     
  11. TheRodDoc

    TheRodDoc Member

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    Model 70 winchester serial no. 104xxx
    220 swift
    varmint barrel 26" with muzzle brake
    target stock from winchester
    bedded barrel and action
    bought new in the end of 1948

    Fecker 10 power scope
    original sling

    This gun was one of two bought at the same time but the other was sold in the later 50's.
    Both had the same scopes and same work on them.

    [​IMG]


    View attachment 117003

    View attachment 117004

    View attachment 117005
     
    Last edited: Aug 27, 2011
  12. TheRodDoc

    TheRodDoc Member

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    I'm the second owner. First was my Dad.
    The metal parts are 100% finish yet. No rust any where at all. Like new. Stock is original finish and slightly lightly scuffed here and there but close to new condition. No varnish flaking off or wore off.
    Bore is in like new condition.

    Trigger is winchester but worked on. Very crisp. You can't hardly tell you moved it. And super light. I think I can almost blow on it to set it off.

    The scope is also like new in the finish department with only one part broke and lost. The front stop ring which Dad replace with a small hose clamp. Original scope caps still with it.

    Going to see about fixing that.
     
    Last edited: Mar 4, 2010
  13. twofifty

    twofifty Member

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    What a classic beauty!
    I'd forgotten that early scopes did not have internally adjustable reticles.
    That scope recoil spring put a smile on my face.
    Nice that it shoots so well, just like it did for your dad in 1948!
     
  14. Offfhand

    Offfhand Member

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    RocDoc, your Swift is a classic varmint outfit, very nice. Many years ago when I was into smallbore shooting I knew a number of top competitiors who used Fecker scopes and said they were the best.
    As to measuring groups, I'm attaching three quick pics of the instrument mostly used in registered benchrest matches. First is overall view on target, showing dial, magnifying lens, etc. Second is without magnifier, more clearly showing overlay with four different caliber circles. Last shows target holes thru magnifier. Note how the two closely overlapping bullet holes are clearly seperated by the fine lines of caliber circle.
     

    Attached Files:

  15. TheRodDoc

    TheRodDoc Member

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    Offhand,

    Good camera work. Nice and clear.
    That really helps me understand how it's done. Neat tool. Thank you very much.

    will have to get one of those.
     
  16. TheRodDoc

    TheRodDoc Member

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    After looking at the pictures of the measuring tool I was thinking that it might be better to have one that the circle is part of the caliper jaws. Made just for the caliper you are using. Thin glass for each jaw with the hole size etched on each that overlap each other when the caliper is closed. When closed both etched holes would be exactly on top of each other. So looking through the magnifier it would look like one hole.


    Then as you line up one side of etched circle on one side of the bullet hole you could slide the caliper open to make the other etched hole line up with the other side of the bullet hole. Then directly read the measurement.

    Or one half hole in a split glass that opens up as a normal caliper would. when closed making a perfectly round hole in glass. Edges of glass butting together like regular calipers.
    I drew a picture of how it might work.

    I would think this would be an even better tool.

    View attachment 117104
     
    Last edited: Aug 27, 2011
  17. Offfhand

    Offfhand Member

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    RocDoc, you have the general idea how it works, but why two circles when only one is needed? To explain further: beginning with caliper closed-on"0", the two widest holes are identified and intersected with the straight line on clear plastic plate. (See picture). The appropriate size circle is centered over the leftmost hole. Then small, rather sharp spring loaded pointer(housed in unit) is pressed into the target so the main unit is locked into position.The reason for pinning the the instrument is that it is vital that it does not move! Then, following the line, the movable part of the instrument is moved to where the same circle centers over the opposite widest hole. Then the measurement is read, it all takes only a few seconds. The instrument shown has circles for four different calibers, making it quite flexable for general use.
     
  18. TheRodDoc

    TheRodDoc Member

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    Ok. Didn't realize there was a pin to hold it place. Kool. Took a little bit but I have it now.
    Thanks again.
     
  19. Hud

    Hud Member

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  20. TheRodDoc

    TheRodDoc Member

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    That looks neat too. I downloaded it.

    I didnt think scanners copyed that accurate but I guess that it is very easy to test it.

    A person can just draw circles with a drafting compass exactly .224" dia. And exactly 1" apart and maybe another 1/2" from one of those center to center then copy and try it. If the software gives the same dimentions then one will know if it is accurate.
     
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