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Adjustable rib question

Discussion in 'Shooting Related Threads' started by gordy h, Sep 19, 2009.

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  1. gordy h

    gordy h Member

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    Will an adjustable rib change the POI? If not what is the reason for it? If
    a adjustable comb will change POI why both?
     
  2. hmb

    hmb Well-Known Member

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    It will change the POI if you change the height of the front sight and when used in conjunction with an adjustable comb allows you to maintain the same sight picture. HMB
     
  3. AveragEd

    AveragEd Well-Known Member

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    Also, if you have a combo and your O/U barrel does not have an adjustble rib, having one on the single barrel allows you to use the comb to get the POI you want with the O/U barrel and the rib to get the adjustable barrel shooting to the same point. That way, you don't have change the comb when you change barrels.

    Ed
     
  4. andybull

    andybull Active Member

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    The reason for an adjustable rib, is to raise or lower the point of impact. I feel that the rib along with an adjustable comb, will allow the full range of adjustment, maintaining the commonly used figure eight picture of the center and the front bead. Lowering the front of the rib and raising the comb, will raise the point of impact, conversely, raising the front of the rib and lowering the comb, will lower the point of impact, again maintaining the same sight picture over the rib.

    If the comb adjustment is done alone, there will be a change of impact, but the sight picture over the rib will not remain the same thru out the up and down spectrum of the comb.

    Andy
     
  5. short shucker

    short shucker TS Member

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    Butch,

    As I understand things, Krieghoff's actually change the pitch of the barrel thus changing POI without adjusting the comb or changing the sight picture.

    All the others use the comb to change POI and adjust the rib to get the desired sight picture.

    ss
     
  6. AveragEd

    AveragEd Well-Known Member

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    BUT if the front of the rib is what moves, when you move it to get that precious sight picture back (who looks at the sights on a shotgun, anyway?), you are double-adjusting and will spend your time wrenching on the rib and comb. Remember, any time you move either sight (your eye or the bead), you change the POI.

    I know that numerous gun owner's manuals tell you to do that but think about the sights-moving thing for a minute before you tell me I'm wrong. Better yet, try it with your gun.

    Back when the Browning Plus guns came out, I watched two of this area's best shooters adjusting their hearts out - too low, now too high, now too low, $hit, now too high - until they realized what they were doing and left the rib alone.

    Ed
     
  7. gordy h

    gordy h Member

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    Thanks for the comments,I learned a few things. now back to the drawing board. Butch H
     
  8. Paladin

    Paladin Well-Known Member

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    If you shoot a Kolar Trap Combo with a fixed rib on the O/U, the procedure is to adjust the comb to get the desired POI for the top barrel, which is the second shot of doubles.

    Then you adjust the bottom barrel POI by changing the hanger between the barrels, which changes the spacing between the barrels. (Leaving the comb alone).

    Then for singles, change the adjustable rib to get the desired POI. (Leaving the comb alone).

    Finally, for handicap, a small upward adjustment of the comb may be needed to get the desired POI. But then, maybe the comb won't need to be adjusted at all, depending upon the shooters technique.

    Often times, all three events can be shot with a Kolar T/C without making any adjustments.
     
  9. jimrich60

    jimrich60 Member

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    Averaged is 100% correct on this. You must remember that in a shotgun, your eye is the rear sight. So just as in a rifle or pistol, raising the comb (and thus raising your eye/rear sight) will raise the POI. For purposes of changing the POI, the only thing that counts is the relationship/angle between the rear sight/your eye, and the front bead. The middle bead must be ignored in this relationship. Its only real purpose in the first place, was (and still is) to allow for the initial shouldering/alignment of the gun, to prevent canting. Since the "line of sight" runs from your eye (rear sight) to the front bead (front sight), when raising that "rear sight/eye, you are of necessity changin that angle, making it steeper. This in turn will make more of the rib visible, and the apparent space between the middle bead and front bead will/must increase. If you somehow force your face more into the stock, or raise the front of the rib to restore the (by now mthyic) figure 8 alignment of the middle and front beads, you have simply negated the raise in POI you were trying to raise in the first place. Think it through. The POI is established by the rear sight (eye) and the front sight (bead) just like in a rifle. The middle bead does not count in this geometry in any way. The only way "keep" the mythical figure 8 bead relationship in the process would be to raise the middle bead to match the new angle between rear and front sights, something you cannot do normally, although a very few shotguns do have adjustable ribs which are pivoted at the front (bead area) and in which you can raise the rear of the rib without raising the front. In this case, you could raise the rib rear (and of course the middle bead would also raise) without changing the front bead height to allow the shooter to maintain the previous middle/front bead relationship (or figure 8 if you like). But such ribs are not the norm.

    On the average (and this is not set in stone, depending on gun, shooter, etc) raising the comb 1/16th inch in theory raises the POI about 3 inches at 40 yards. For many guns with adjustable ribs, lowering the front of the rib one notch or mark typically lowers the POI about 1 inch (this all assumes a 34 inch barrel). Thus if you raise the comb 1/16 (one typical spacer/washer) and then lower the rib front one mark/notch, you will have a change (upward) of 4 inches in POI. Or you could raise the comb and thus POI by three inches, raise the front the rib by one notch, and have a resulting 2 inch raise in POI.

    Gross changes in POI should be set through comb height changes. Then you can "fine tune" the POI through additional adjustable rib changes. Or, as Averaged mentions, if you have a combo, first use the comb to set the POI on the normally non-adjustable rib o/u barrels. Then change to the single barrel with adjustable rib, and use the rib the adjust the POI of the single barrel to what is wanted. Then you can use the stock/action with either set of barrels without having to make any changes in the stock between barrel changes.

    Jim R
     
  10. zzt

    zzt Well-Known Member

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    Butch, I have to disagree with some of what has been said here. Adjusting ribs and/or combs is not rocket science. Just remember that when you adjust the rib AND the comb, the changes in POI are additive.

    I have an MX-2000RS combo. It has a tetter-totter type adjustable rib. Each notch changes POI at 40 yards by 1.4". The manual says that if you add or subtract a 2mm spacer for every notch you move the rib, you will have the same bead alignment and POI will move 4" at 40 yards. The manual is correct.

    Adjusting just the rib moves the POI 1.4" @ 40 yards for each notch. Adding or subtracting 2mm spacers under the comb changes POI by 2.6" @ 40. So if you want to fine tune your POI and don't care about seeing the same mid/front bead alignment, you can move POI in small increments by adjusting either the rib or the comb. If you must see the same bead alignment, you must move both together. In that instance, the smallest adjustment you can make is 4" @ 40.

    Dimension for other guns may vary, but the concept is the same. You can move the bead, or the comb, or both together.
     
  11. CalvinMD

    CalvinMD Well-Known Member

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    All i know is if I turn d lil screws one way the pattern gores up...the other way down...so.yes
     
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adjustable rib for trap shooting

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does the line of sight alter with the adjustable rib when raising height