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Adjustable Ribs and Combs

Discussion in 'Shooting Related Threads' started by AEP, Aug 2, 2009.

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  1. AEP

    AEP Member

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    May 6, 2009
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    When do you adjust the rib and when do you adjust the height of the comb?

    From reading past posts it seems if you raise the comb up or down raises or lowers the POI.

    Can someone please explain?

    Thank You,
    Andy
     
  2. threedeuces

    threedeuces TS Member

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    They both raise or lower point of impact, so take your pick.
     
  3. chiefjon

    chiefjon Active Member

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    I would offer this. The comb is about gun fit and POI. The rib is about sight picture and to a much lesser degree POI. When your face is on the comb and your looking flat down the rib and you LOP is right then you can find out where the POI is. You can adjust that to some degree by adjusting the rib. You must then imprint the bead alignment in your brain to make sure you are mounting the gun the same every time. The POI should then follow your eyes if you mount the gun and do not move your head from the stock. IMHO

    JON
     
  4. jimrich60

    jimrich60 Member

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    The best advice I got on this came from Averaged (and matched instructions from Beretta for good measure). First adjust (raise, lower, move right or left) to set the basic desired POI (and ensure POI is centered left/right). Then , if needed or desired, use the rib adjustment to "fine tune" your POI. Basically, each 1/16th change (one washer) in comb height will move the POI about 3 inches (nominally, as every person will see some differences of course). At least on my Beretta, per Beretta, each notch of the rib adjustment gives about 1 inch change. So set the comb for best shooting, then tune with the rib if needed. This applies even more to combo sets, where the rib on the o/u barrels is not adjustable. Set up the comb for the o/u barrels, then set the single barrel to shoot to the same POI by adjusting the rib.

    Jim R
     
  5. zzt

    zzt Well-Known Member

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    Andy, what you adjust, and how, will be determined by your preferences.

    If you like the sight picture Chiefjon suggests, flat down the rib (buried beads), you will absolutely have to adjust both the rib and the comb together. The same is true even if the mid/front bead alignment you prefer is a figure 8, or something else. The simple fact is that if you move one but not the other, the bead alignment will change.

    I'm not bothered by changes to the bead relationship. I almost always see some rib between the beads, so a little more or less does not matter (to me). I often adjust my rib and comb separately, depending on how much change I want. If I move my Perazzi's rib one notch and leave the comb alone, I change my POI by a little more than 1.5" @ 40 yards. If I add or subtract approx. 2mm of spacers under the comb to match the new rib position so I see the same bead alignment, POI moves a total of 4" @ 40 yards.

    Just remember the changes are additive. With my setup I can fine tune POI by adjusting the rib one notch for a 1.5" change in POI @ 40 yards. Or, I can leave the rib alone and add 2mm of spacers under the comb to move POI by 2.5" @ 40 yards. Alternately, I can make both adjustments at the same time for a 4" change @ 40 yards.
     
  6. AveragEd

    AveragEd Well-Known Member

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    Just remember that when you move either the comb OR the front of the rib, you are changing the gun's POI. Some gun owner's manuals tell you to adjust one of them to arrive at the desired POI and then adjust the other to get your sight picture back - that's "double adjusting" and you will chase your POI all over the place that way. A caveat - if only the BACK of your rib moves, you can adjust it without affecting POI.

    A "figure-eight" sight picture is not required and I've found that many whose guns only shoot where they want them to with a gap between the beads but do not have an adustable rib they can use to reduce or eliminate that gap learn to like that sight picture better as the gun is out of your field of view that way.

    As Jim said above, use your adjustable comb to get your fixed-rib barrel shooting where you want it to and then adjust the rib on your barrel with that provision until it also shoots to that same point of impact. That may leave you with a gap between the beads on both barrels (mine is like that) but that's unimportant. Adjusting the rib to get rid of that gap with that barrel means you will have to change your comb setting every time you change barrels. That pretty much defeats the purpose of an adjustable rib and you WILL forget to do it now and then. I used to have a combo with fixed ribs on both barrels; the barrels shot to different POIs and I forgot to move the comb more often than not.

    Ed
     
  7. jimrich60

    jimrich60 Member

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    The best advice I can give on adjusting the comb for POI is to ignore that middle bead entirely. POI is determined by relationship of the front sight (bead) and the rear sight (your eye). As you raise the rear sight/eye, in relation to a fixed front sight/bead, the POI will move up. But as you do that, you will necessarily see more of the rib between your eye and the front bead. If you then try to once more align yourself to see the middle bead and front bead in a figure 8 alignment, you are forcing your eye back down, thus forcing POI back down. The only way to avoid this "loss of the middle/front bead alignment would be if you had a rib which was adjustable at the rear. In such a case, you could raise the rear of the rib (which also raises the middle bead) until you once again saw that figure 8 alignment since you would not, in such a case, be changing the alignment of the rear sight/eye and the front sight/bead. Most adjustable ribs, however, only adjust at the front, and changing this will again change the POI. If you, for instance, raise the comb, and then raise the rib/front bead to see your figure eight again, you have just cancelled out the change in POI you achieved by raising the comb in the first place. On the other hand, if you lower the front bead/rib after raising the comb, you are "adding to" the increase in POI change upward. I have seen a lot of people who first change their comb because they say want a higher POI, then immediately start to fiddle with the rib to achieve the "same sight picture" (whether a figure 8 or other middle/front bead relationship) as they had before, thus completely negating the raised comb and increase in POI they originally sought. Then they could not understand why the changes were not reflected in they way targets broke (or did not break) for them. I think the best way to think of POI adjustments is to think in terms of rifle sights. Raise the rear sight (your eye for a shotgun) or lower the front sight (front bead) to raise the POI or vice versa. There is no "middle sight" in this equation, and for all practical purposes, that middle bead should be ignored in setting up a shotgun POI as well.

    One point I should make, is that the middle bead does serve a couple of purposes (just not in setting up POI). By aligning the middle and front beads vertically (regardless of whether this is a neat figure 8 or whether they are separated by some distance) they do help avoid canting of the shotgun, as well as ensure that the rear sight (your eye) is centered down the rib (thus ensuring the cast-on/cast-off is properly set for you. In my opinion, this is the only legitimate function for a middle bead, and it really only applies to target guns where you pre-shoulder the gun. I would note that hunting shotguns, which of course, are not pre-shouldered, do not have a center or middle bead since it would serve little purpose in a quick shouldering movement. Of course, once you have aligned the shotgun and set yourself to call for the target, neither of the beads should consciously appear in your vision or be used to "sight" the gun. Focus at that point should be solely on the target.

    Jim R
     
  8. timb99

    timb99 Well-Known Member

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    Raise the comb = higher POI<br>
    Lower the comb = lower POI<br>
    Raise the front bead = lower POI<br>
    Lower the front bead = higher POI<br>
     
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