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poi again

Discussion in 'Shooting Related Threads' started by Traders, Nov 4, 2010.

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  1. Traders

    Traders Well-Known Member

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    i know that the shooter's height of eye, controlled by the comb height determines the POI. What I don't understand is what the rib has to do with it. Seems to me that adjusting the rib height will change the relationship and appearance of the distance between the the front and middle beads but unless the comb is moved, will have no effect on the POI. I know some people seems to think that adjusting the rib is a form of "fine tuning". I can't see that it has POI effect.
     
  2. dverna

    dverna Active Member

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    Changing rib height will not affect POI.

    Changing the angle of the rib will affect POI.

    Don Verna
     
  3. Neil Winston

    Neil Winston Well-Known Member

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    Changing the rib height will affect the POI. If the angle is the same and you compensate with an equal change in where your eye is, vertically, the change won't be much. But it can affect what you conclude from close-range testing if it is a lot different from what you are used to.

    Neil
     
  4. jimrich60

    jimrich60 Member

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    The problem lies in your definition of "changing rib height". With an adjustable rib, you are in reality, changing the height of the front bead in relation to the rear sight (your eye). Just as in a rifle, you can change the POI by either changing the height of the rear sight or the front sight (to raise impact, raise the rear sight or lower the front sight, for instance). Same applies to a shotgun in general terms. Has nothing to do with the middle bead per se, or the relation of middle to front bead. It really just boils down to the angle between the rear sight (for a shotgun, your eye) and the front sight (the bead). So, raise the comb (thus your eye) and raise POI, or lower the front bead, still raise the POI. Adjusting the rib can "fine tune" the POI simply because the rib adjustment increments are normally smaller than the increments (usually by washer) that are used for the comb. As as example, raising the comb by one washer (1/16 inch) will nominally raise POI by 3 inches, while one notch or increment of lowering the front bead (adjusting the rib) will raise POI by one inch. So the rib can be used to make smaller POI changes normally than the comb.

    Jim R
     
  5. hmb

    hmb Well-Known Member

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    Jim R has it right. Also you can adjust the rib, without changing the height of the front bead, this will give you the desired sight picture. HMB
     
  6. jimrich60

    jimrich60 Member

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    HMB

    Just curious. What adjustable rib can be moved without changing the height of the front bead (since that is the purpose of all adjustable ribs I know of)? Is there an actual rib that does this, or were you just referring to the idea that if you could leave bead height alone and just raise the rear of a theoretical rib to change "sight picture" you would not change POI. Again, you have my curiousity if such a rib exists on the market.

    Jim R
     
  7. dverna

    dverna Active Member

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    Neil,

    You absolutely correct. But in practical terms have not helped the man understand.

    An addition of a 1/2" rib parallel to the old rib will lower POI by 1/2" (at all distances). At 35 yards, it is a "theoretical" change; but not discernible by a shooter.

    Changing the slope of the rib by 1/2" will change POI by about 16" at 35 yards (assuming a 40" sight radius).

    Trader,

    Draw a sketch to scale and see what happens. It is simple geometry and common sense. Adding a higher parallel rib just make the gun shoot lower (if you raise the comb to get the same bead separation). We need to point the end of barrel up at a greater angle to get the POI up. Lowering the front sight or raising the comb does that.

    Don Verna
     
  8. Twinbirds

    Twinbirds TS Member

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    didn't my K80 pull the un single barrel up? the rib remained fixed? been so long I may not remember correctly
     
  9. senior smoke

    senior smoke Well-Known Member

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    This is getting way to complicated for me. Years ago I would tear off a box top or add mole skin to raise my poi. Then we went to adjustable stocks, which I agree is a great for getting your gun to shoot where you want it to shoot. BUT, adding an adjustable rib now to the mix is to much for my pea size brain to comprehend. Since adjustable ribs first came out I have seen shooters start making adjustments and get their gun shooting why out of wack. Some could not even get the gun to ever shot back the way they originally had it shooting. Adjustable stocks, adjustable ribs are great for people who know and understand how to really use them, and don't continually make adjustments every time they miss a target. For this dunderhead, I will continue to just tear off a box top off a box of shells and tape it to my stock or place it under a piece of mole skin. Life use to be so much simpler than it is today.
    Steve
     
  10. phirel

    phirel TS Member

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    Don Verna- Both you and my High School Geometry teacher claim that two parallel lines will never converge or diverge. I agree with both unless we want to get into space/time warp, but none of us shoot at those distances.

    Pat Ireland
     
  11. Rastoff

    Rastoff Active Member

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    Just to confuse things more, I thought I'd throw this pic in here:

    POIwithmathsmall.jpg


    This is a pictorial representation of how much your POI will change based on a change in comb height. The same ratio works for moving your comb left or right.
     
  12. 3357

    3357 Member

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    My KX-5 instruction manual advises to "adjust the rib to obtain the desired point of impact, then adjust the comb to obtain the desired sight picture and you are ready to go" fyi only.
     
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