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how does an adjustible rib change POI

Discussion in 'Shooting Related Threads' started by knutershooter, May 14, 2012.

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  1. knutershooter

    knutershooter Member

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    How does or can someone explain the adjustible rib setup to change POI? I Have read that the way to change POI is with the comb and that the adjustible rib allows you to keep your sight picture the same. Can you also raise or lower the POI by adjusting the rib without raising or lowering the comb.

    Thanks!
     
  2. Beretta687EELL

    Beretta687EELL Well-Known Member

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    The adjustable rib allows you to raise and lower the POI by adjusting the front of the rib up (to lower POI) or down (to raise POI). The sight picture stays the same, at least it does on my Kolar T/A Max. Bill Malcolm
     
  3. knutershooter

    knutershooter Member

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    This is on an XT Trap combo and the screws are in the middle, then you can adjust the rear of the rib up or down also.
     
  4. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    Actually,moving the cheekpiece will raise or lower your dynamic point of impact. If you shoot off your front bead then raising or lowering it will change your "sights". Remember on shotguns the rear sight is your eye.

    Adjust like a rifle...
     
  5. zzt

    zzt Well-Known Member

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    I'll expand on the two good answers you already have. Before I do let me say that what follows does not apply to a standard K-80 unsingle, because adjusting the "rib" is actually bending the barrel. It also does not apply to ribs that pivot at the bead.

    Most adjustable ribs pivot in the center (tetter-totter) or at the receiver end of the rib. As such, any adjustment you make to the rib moves the bead relative to where it was. When you move the bead, the shot goes in the opposite direction as Bill states.

    Moving the comb in any direction also moves the POI, but the shot moves in the same direction as the comb moved.

    So if you can move the rib (bead) to change POI or you can move the comb to change POI, why have both. The reason is simple. If you care a lot about always having the same bead alignment, you have no choice but to adjust the rib and comb together.

    Here is an example of how it works. Let's say you like to see buried beads when you look down your rib. That is, the beads coincide and you can only see the outline of the front bead around the center bead. You decide to raise your POI so you adjust your rib so the bead is lower. Now when you mount the gun you do not see buried beads. In fact, you probably can't even see the rib.

    Lowering your front bead by adjusting the rib raised your POI but altered the bead alignment you prefer. To get it back you have to raise the comb by a proportional amount. Raising the comb also raises the POI, so you raised POI a little by adjusting the rib and a little more when you adjusted the comb to get your sight alignment back to where you wanted it.

    This is what so many find confusing about adjustable ribs and combs. They work in conjunction with one another.

    Here is a real world example. The directions that came with my Perazzi suggest you add or remove 2mm worth of spacers under the comb for every notch you adjust the rib. So lowering the front of my rib one notch raises POI and adding 2mm of spacers under the comb also raises POI but brings the beads back to the same alignment. Adjusting the rib accounts for about 1/3 the total POI change, with the comb adjustment accounting for the other 2/3. That ratio is not the same for all guns and is really dependent on where the pivot point of the rib resides.
     
  6. 12ShotTwo

    12ShotTwo Member

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    zzt -- Good explanation, I was wondering at this on my KS-5 Special but I have 1 question. Does your Perazzi pivot on the receiver like my KS-5? Joe
     
  7. knutershooter

    knutershooter Member

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    Ya the directions on the browning are clear as mud and I have never had an adjustable rib before. I was told that changing a 1/16 on the comb and a 1/16 on the rib would in effect change it a total of 1/8. I wanted to try a 32nd lower and was hoping not to have to change the comb but maybe lower the rib a 32nd or wait raise it a 32nd, does the moon pahase have anything to do with it?? LOL!!!
     
  8. AveragEd

    AveragEd Well-Known Member

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    I know it sounds dumb to suggest that the gun makers don't know this but following their instructions for adjusting POI often leads to a frustrating game of hide-and-seek with your gun's point of impact. Adjusting a comb to change the POI and then adjusting a rib that pivots in the rear or center is double-adjusting. Moving either one changes POI so if you raise the comb to raise the POI and then lower the front of the rib to regain a figure-eight sight picture, you will move the POI higher yet.

    Yes, you can spend the time it takes to MAYBE find perfect spots for the rib and comb that yield the right POI and the desired sight picture at the same time but that's not always possible. And then, if your gun is a combo with an adjustable rib on the single barrel and a fixed rib on the O/U set, all that goes for nothing when you change barrels, as the non-adjustable set isn't going to shoot to the same POI as the adjustable one.

    It's this simple. Like all firearms with sights, your shotgun has two - the front bead and your eye. And like all firearms with sights, move either one and you change POI.

    Quite often, the figure-eight sight picture is not obtainable with the desired POI. That's just how it is and it really doesn't matter as that center bead really serves no sighting purpose. In fact, I think it lends a false sense of security to gun fit. If you mount your gun and move your head down or up until the beads are in a figure-eight before calling for a target, you only have temporarily aligned the beads because as soon as you take your eye off the beads as you and your gun start moving toward the target, your head will return to its natural position and the beads will no longer be aligned in that all-so-important figure-eight. If you have to force it to happen, you're only kidding yourself.

    Remember the two most important rules in shooting a shotgun - your gun has to shoot where you look and you have to look at the target. It doesn't matter if there is a gap between the neads or not or if the center bead is even there at all. In fact, a gap helps move the gun down away from your brain's view of the front bead through your eye.

    Back when the Browning Plus guns came out, I watched two ATA All-Americans and state champions shooting their BT-99 Pluses for the first time. With the book in one hand and the wrenches in the other, they shot and adjusted for hours, never being able to get the guns shooting where they wanted. They moved the comb until the POI was right then moved the rib until the beads aligned, after which the POI was wrong again.

    Soon afterward, I bought the first Citori Plus to come into the area strictly because it was a non-ported 32" gun. As I read the POI adjustment portion of the instructions, I realized what those two men were doing. Neither kept their guns long, saying they coud never get them to shoot where they wanted them to. I guess not - so much for following instructions, huh?

    Ed
     
  9. 12ShotTwo

    12ShotTwo Member

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    What I thought was interesting was that zzt said that moving the rib contributes 1/3 and the comb 2/3 given that you move both the same physical amount in the proper directions. I, maybe like most, thought that they contributed equally. Joe
     
  10. RickN

    RickN Well-Known Member

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    Think of it like an iron sighted .22 rifle. Use the two beads same as iron sights.

    Adjust the sights, whether front or back, then place your face on the stock so you look thru the sights.

    Now adapt that to your adjustable rib-adjustable stock shotgun.

    Adjust the sights.....the rib, so it shoots where you want it, then move the stock so you're looking straight thru the sights....down the rib.

    No more complicated than that.
     
  11. AveragEd

    AveragEd Well-Known Member

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    Moving the comb has a greater effect on POI than the rib. I like to get my guns "in the POI ballpark" with the comb and then fine-tune with the rib.

    Ed
     
  12. Dr.Longshot

    Dr.Longshot Banned Banned

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    Both need adjustments to co-incide with each other to keep everything in perspective.

    Gary Bryant
    Dr.longshot
     
  13. AveragEd

    AveragEd Well-Known Member

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    Could we have a more detailed explanation of how that works?

    Ed
     
  14. darr

    darr Well-Known Member

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    Adjusting the comb makes most of the POI change.By raising the comb you are in effect lowering the butt of the gun thus making it shoot higher.Adjusting your rib only changes the sight picture.The gun shoots where it shoots.By lowering the front bead you can have a sight picture that you would with a high shooting gun.If you are a died in the wool bead checker the rib may make a difference but if you are watching the target it shouldn't make much.I can't tell you where my beads are when I break a target.From what I have read that is the way it is supposed to be done.I am by no means a great shooter ,but it has taken me about a year to get off the beads and adjust my gun to shoot where I look.Moving your comb makes 95% of the change IMHO

    Darr
     
  15. Stl Flyn

    Stl Flyn Well-Known Member

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    I never look at my beads. To be honest with you it would be kind of interesting if someone would take my bead off without me knowing, and see if I would notice, or if it made a difference. IMO, I think the comb makes the biggest difference, with the slightest amount of adjustment. I also think that the rib adjustment would just change the sight picture when mounting the gun. After that I am not sure it makes a difference when you are concentrating on the target. Just my opinion.
     
  16. AveragEd

    AveragEd Well-Known Member

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    If you move the front bead (sight) down, you in effect raise the muzzle, elevating the POI. If you move the front bead (sight) up, you in effect lower the muzzle and the POI. Sight picture has nothing to do with POI - sight alignment does. Move either your eye (the rear sight) or the bead (the front sight) and you move the POI. There is no "maybe" about it; it's fact.

    If you can't grasp that concept, draw a barrel with a line out of the muzzle on a piece of paper with the barrel level. Now tilt the paper as if you raised the rear sight - keep the sights level - and the line will impact higher. Do the same thing with the front sight and the line will impact lower. Better yet, perform that experiment with a small-caliber rifle or handgun with an adjustable rear sight. "Raise" the front sight by placing a dollop of chewing gum or putty on it or lower it by removing that material. Change the height of either sight and your bullets WILL impact the target at different elevations.

    Ed
     
  17. zzt

    zzt Well-Known Member

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    12ShotTwo, my Perazzi rib is a teeter-totter type with the pivot point in the approximate center of the rib.

    The 1/3, 2/3 ratio I mentioned is only for my gun. Remember, I said it would depend of the pivot point of your rib. With your rib pivoting at the receiver end, your ratio is more likely to be closer to 50-50.

    Also bear in mind the examples I was referencing involved a situation where you wanted to see the same sight alignment after the adjustment. I'm not bothered by such things. In fact, I have a clearer view of the target if I see a little rib. So adjustments for me are simple. When I want to make a minor adjustment, as I sometimes do on the line, I adjust only the rib. If I want more of a change I adjust only the comb. For major changes I can adjust both.

    The geometry of this is simple when you stop to think about it, but difficult to explain clearly because of the different pivot points.

    Let me give you another example, along the lines of what AverageEd said. On my Perazzi, if I insist on seeing the same bead relationship after I adjust POI, I have no choice but to adjust the rib AND the comb. Those adjustments are additive, so the smallest increment I can adjust my POI by is 4". That's a lot. Too much as far as I'm concerned, and why I adjust as I mentioned above.

    I can adjust my rib to give me a total of 4.5" of POI adjustment in 1.5" increments. That's plenty fine for just tweaking. In practice, I leave the rib in the middle of the adjustment range to give me the ability to adjust 1.5" up or down. For any changes greater than that, or for permanent changes, I adjust the comb. That way I can always make a temporary change using the rib.
     
  18. 12ShotTwo

    12ShotTwo Member

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    Thanks zzt. I also set my rib in the middle of its range, set the comb for smoke @ singles and tweek the comb a little from there for hcp. Thanks Joe
     
  19. dead on 4

    dead on 4 Well-Known Member

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    An adjustable rib is nothing more than a method to adjust the barrel up or down high/low relative to your sighting plain which is the top of your rib.

    If you hold your gun with your rib level and your barrel is parallel with the top of your rib, you gun will shoot flat 50/50 + or -. When holding your gun level and you pull the front of your barrel up as high as it will go maybe touching the bottom front of your rib, your gun will shoot its highest point of impact because the barrel is no longer parallel to the top of your rib, its pointing upward relative to your sighting plain. If you push your barrel down and away from your rib, your gun will shoot lower relative to your sighting plain.

    Its the taper from rear to front that determines how high a gun will shoot. Some gun companies have the taper adjustment at the front of the barrel, others raise or lower the rear of the rib to create the taper which determines
    if the gun will shoot high or low relative to the sighting plain.

    In most but not all cases, when you adjust your rib the comb may need adjusting up or down. The front/mid beads serve as a reference when adjusting your rib allowing you to keep the same sighting plain, they also allow you to recover quickly if something gets out of adjustment.

    When you raise your comb only, you loose the top of the your rib as a sighting plain and your barrel target relationship starts and ends at the front of your barrel, nothing wrong with this, its just a different method of putting the shot on target, some people are bothered by the gap created by raising the comb only, but that's where the bump comes into play...........

    Surfer
     
  20. goose2

    goose2 Well-Known Member

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    I shot some of my best scores with "both" beads removed.
     
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