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Adj. Comb Height Change vs. POI Change

Discussion in 'Shooting Related Threads' started by GW22, Dec 6, 2012.

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  1. GW22

    GW22 Active Member

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    Refresh my memory please - what's the rule-of-thumb for the ratio between comb height change and POI change? Isn't it 1/8" = 3" at 35 yards or something?

    Thanks,

    Gary

    PS: Yes I realize it needs to be verified on the patterning board :)
     
  2. Star4Ever

    Star4Ever Member

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    Hi Gary:
    From the 16Y line, for every 2" aprox (corrected) you want the center of your pattern to move you move your comb 1/16" or .0625". This fact (information) is in the "Patterning For Perfection" document by Pete Blakely.

    BTW, to raise the pattern raise the comb (or lower the bead), and to lower the pattern, lower the comb (or raise the bead).

    Hope this helps,

    Art
     
  3. GW22

    GW22 Active Member

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    Thanks Art. Doesn't that seem a slightly low, though? So 1/2" of comb adjustment only moves the pattern 8"? I've done a lot of patterning but I didn't keep notes on the comb adjustments used to get where I was going.

    -Gary
     
  4. 9point3

    9point3 Well-Known Member

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    Art,

    Too many variable to list your rule as "fact"

    Eye to bead distance has some to do with it. (changes with barrel length and gun mounting/fit) as well as at what distance the shooter breaks his targets.

    Prolly a good rule of thumb but not "fact"
     
  5. GW22

    GW22 Active Member

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    Art: Click the link and re-read Blakeley's article. I believe he's saying that 1/16" yields 1" when patterning board is 16 yards away -- not when shooti ng 16 yard trap. Right?

    So, if that's the case, the POI rise at 35 yards would be over 2".

    -Gary
     
  6. GW22

    GW22 Active Member

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    Neil! Please HELP!

    -Gary :)
     
  7. Scott Johnson

    Scott Johnson Well-Known Member

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    I remember 1/8" being 4" @ 40 yards, but memory is a fleeting thing. Scott
     
  8. Kemper

    Kemper Active Member

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    What does it do? Push the gun down on your shoulder?
     
  9. GW22

    GW22 Active Member

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    Good point, Bigski - I'll just take a bead-to-eye measurement and trig it out to end the guesswork.

    Thanks,

    Gary
     
  10. wayneo

    wayneo Active Member

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    Scott and Bigski have it. 1/8= 4" at 40 yards. + or - a little for some people. Put it on paper. Put one of those big magnetic Hi-Viz sights on your rib and see how far your POI drops, at least 4" for me. It's about an 1/8" taller than my factory bead.

    Wayne
     
  11. Dickgshot

    Dickgshot Well-Known Member

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    Except it doesn't work like that. You can't translate changes in comb height
    to a change in POI mathematically. When you raise the comb it pushes your head higher. But since your head (hopefully) is attached to your body, it changes the
    angle of your head to the stock. That changes the distance your eye is from the
    comb. It also can change -as Kemper said -the position of the stock in your shoulder.
    And that doesn't take into account gun movement. Do you hold a low gun and come up through the bird or a level gun with no vertical movement?


    The only way to arrive at the right point of aim is trial and error. Look at your breaks. Are you taking off the top of the target or the bottom. That's the only way I've ever seen a good shooter adjust his vertical point of impact.
     
  12. BIGDON

    BIGDON Well-Known Member

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    Different for different folks. The only true place to find out "up or "down is the patterning board. Do it at 13 yds and then multiply by 3.

    Don
     
  13. zzt

    zzt Well-Known Member

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    You absolutely can predict POI changes mathematically. I've posted the formula here and in PMs to members who asked. Here it is.

    Divide your eye-to-bead distance into your target distance (keep measurement the same) to find your multiplier at that distance. Then multiply the amount you raised or lowered your comb or bead to find the POI change at target distance.

    Example: there are 45" between my eye and the bead on my unsingle. If I want to know how much I will change POI at 40 yards I convert 40 yards to inches, 40 x 36" = 1440" then divide by the eye/bead distance 45" to find there are 32 eye/bead units in 40 yards. That is your multiplier. If I change my comb or bead by 1/8", the effect will be 32 times greater at 40 yards. 32 x .125" = 4.0" at 40 yards. At 30 yards the multiplier is 24, so the POI change will be 3".

    Note: this is for my 35" unsingle barrel. If I used my 30" barrels the change would be different. 1440" / 40" (new eye/bead) = 36. So a 1/8" change using my 30" barrels moves POI by 36 x .125" = 4.5" at 40.
     
  14. Dickgshot

    Dickgshot Well-Known Member

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    What BIGDON said, or what I said. Both of those work. Yours doesn't. Sorry.
     
  15. DustyOne60

    DustyOne60 Active Member

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  16. Star4Ever

    Star4Ever Member

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    Guys:
    I have a comb adjustment and point of impact calculator that someone sent to me a while back.
    It is very nicely done by Jesper Greth. Takes into consideration, barrel length etc.
    Is in both metric and yards/feet/inches.

    I will post it on my website.... the URL will be:

    http://www.lithicsnet.com/POI Calculator.xlsx

    I will be uploading it there in a few minutes...

    Art
     
  17. Rollin Oswald

    Rollin Oswald Active Member

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    Anyone,

    Send me an email with "POI Calculator" in the subject line and I will reply with an attachment page of an automatic POI calculator with a drawing and the equations for figuring it out.

    The calculator does the math in the equations, automatically. All you would need to to is plug in the numbers. You do need to have Microsoft Excel on your computer to open and use the calculator, however.

    Rollin
     
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