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Adjustable Rib/Comb

Discussion in 'Shooting Related Threads' started by Mo Bill, May 10, 2010.

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  1. Mo Bill

    Mo Bill Member

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    I have a new MX 15..I am confused about how to get it set up.
    To get the vertical part of my desired POI don't I just adjust
    the comb up or down?...Do I then adjust the rib to get the fig 8?
    I know that seeing space between the mid and front bead is ok..it's
    just that the fig 8 is easier to check with confidence.I do not
    consciously bead check..but..subconsciously I think we all do,and
    the fig 8 is probably more accurate.


    Thanks for your help...

    Mo Bill
     
  2. andybull

    andybull Active Member

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    Adjust your rib and then your comb.
     
  3. Steve W

    Steve W Well-Known Member

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    Your MX-15 has 6 notches with 1/4" vertical movements on the front bead, you really need to adj. the rib according to the factory instruction if you don't test POI on patterning board.

    Figure 8 is a personal preference.
     
  4. AveragEd

    AveragEd Well-Known Member

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    If you adjust the rib until it shoots where you want it to and then adjust the comb, you will be double-adjusting. Any time you move either sight (the front bead OR your eye), you change the POI.

    I would set the rib in the middle and adjust the comb until it shoots about where you want it to. Then if shooting at several hundred targets tells you the POI needs tweaking, adjust the rib accordingly.

    Ed
     
  5. Mark425

    Mark425 TS Member

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    The way I done it, which may be right or wrong is:

    Put the rib in a mid position or as the manual suggests for the approx POI your looking for.

    Adjust the comb to get the POI I want and then adjust the rib to get the desired sight picture. Has always worked for me. Minor tweeking later on was all that was needed.
     
  6. jimrich60

    jimrich60 Member

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    Follow averageds advice. Adjust the comb until you achieve something close to your desired POI. Remember, for most shotguns, moving the comb up or down 1/16 inch (one spacer normally) will give you approximately a 3 inch change in POI (per Beretta). Then you can move the move the rib up or down to fine tune the desired POI, since for many ribs, moving the rib one click or line will generally (again, per Beretta) give you approximately 1 inch change in POI. Comb and rib adjustments on your particular gun may give you slightly different amounts of movement in the POI of course, but in general, the sequence and changes will be about the same for practical purposes. I followed Eds guidance when I first bought my Beretta combo, and they work perfectly. To check POI, and then verify any changes to POI, be sure to test fire the gun from a rest at 13 yards. A POI change of 1 inch at 13 yards equals 3 inches of change at 39 yards, or about 10 percent.

    Jim R
     
  7. phirel

    phirel TS Member

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    Don't you adjust the rib so that the mid bead and the front bead on the comb line up? Or, have I got something mixed up?

    Pat Ireland
     
  8. jimrich60

    jimrich60 Member

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    Pat

    I think that works only if the adjustable rib pivots at the middle rather than at the rear, as most ribs do. The line of sight is your eye and the front bead, the mid bead is totally irrelevant. By trying to re-set the rib so that the mid and front beads "line up" again, you simply end up negating the POI adjustment (at least to some extent) that you made by changing the comb, raising the rear sight/eye in relation to the front sight/bead. If the rib pivots in the middle however, then the rear of rib will move so that as the front bead is lowered (for instance) the rear of the rib will raise up to more or less match the new rear sight/eye to front sight/bead alignment.

    Jim R
     
  9. alfermann66

    alfermann66 Member

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    I think women are right. Men will try anything to avoid reading the directions. Perazzi provides a booklet that tells you step by step how to adjust poi. It is the simplest method I have ever seen and even comes with pictures for the truly dense. 1. Select your model of Perazzi from the booklet. 2. Choose how high/low you want the poi. 3. Position the rib in the notches as instructed. 4. Adjust comb to present a figure 8 bead picture. The gun will now shoot where Perazzi says it will no matter whether you're fat, skinny or other. If you're not hitting targets now it is clear you had no idea of where you wanted the poi. Shish!

    Buz
     
  10. jimrich60

    jimrich60 Member

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    The problem is that even manufacturer instructions are often conflicting. Beretta also gives clear steps to setting the POI, but they are in direct contrast to those for Perazzi. So why do two different guns have conflicting methods, even though both guns work exactly the same way? Since not even manufacturers seem to agree on how to adjust the POI on a shotgun, it really is no mystery why there seems to be so much confusion among shooters.

    Jim R
     
  11. AveragEd

    AveragEd Well-Known Member

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    Buz, while I agree with your thoughts on men and instructions and I hardly ever pretend to know more about something than the folks who designed it, I do make an exception when it comes to adjustable trap guns. Many of their manauls contain incorrect instructions.

    Back when the Browning BT-99 and Citori Plus guns came out, two very good state champion-caliber local shooters bought BT-Pluses and I bought a Citori Plus. I arrived at a local club to find those two shooting and adjusting over and over while going from too high to too low and back to too high and on and on. Thinking I'd better not try embarassing myself as they were, I dug out my trusty manual and realized the cause of their frustration.

    Browning, like a lot of manufacturers, tells you to adjust the rib to arrive at the desired POI and then move the comb to get your sight picture back. Following those instructions will cause you unending mental overheating because you will be double-adjusting. Think about it - if you move the front of the rib down to raise the POI to where you want it and then move the comb up to get your sight picture back, you just raised the POI even more because you moved the rear sight up. The same thing is true going the other way.

    I will offer the caveat that this is true with every adjustable-rib gun I've ever owned but that I've never owned one with a "teeter-totter" rib. Mine have all been hinged at the rear.

    I have absolutely no idea why those manuals are written like that unless it is because they are written by engineers who never use their products for their intended purpose.

    Ed
     
  12. Rastoff

    Rastoff Active Member

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    If you use the figure 8 you will not be "double adjusting" and it doesn't matter what kind of rib you have.

    If Perazzi says that setting the rib in the middle gives you a POI of 70% high (I don't know what the actual numbers are) then you adjust the comb to get a figure 8 with the beads, then it will shoot 70% high. No patterning necessary.

    I'll add my own caveat here, everyone is different. You should pattern anyway.
     
  13. AveragEd

    AveragEd Well-Known Member

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    So it doesn't matter where your eyes are located in your head in relation to your checkbone, a Perazzi with the rib set in the 70%-high notch WILL shoot 70% high. In other words, the position of the rear sight means nothing?

    Forget the math - just think about how sights work, draw some lines, do whatever you have to in order to visualize a line of sight through two sights and a line representing the bore and the path of the shot. Let's imagine that the lines are fairly parallel and the gun shoots too flat. Move the front sight down to get the gun shooting 70% high. Now move the rear one (your eye) up to restore that figure-eight sight picture. If you understand how sights work, I guarantee you will agree that your gun will now shoot more than 70% high.

    Ed
     
  14. Rastoff

    Rastoff Active Member

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    Let me see if I can say this differently.

    Of course your eye is part of the equation. But, if you're using the figure eight, where your eye is positioned is controlled by the rib.

    I think the disconnect we are having is that your thinking the POI is set without the stacked beads and then you stack the beads. That's not the case in this scenario. In this situation you don't have the X% unless you have stacked beads. Unless I'm wrong, and that's happened plenty, the instructions Perazzi is using is based on stacking the beads. Thus, it will be whatever they say when you get the beads stacked.

    Like I said before though, everyone is different. Your stacked beads and mine will be slightly different, but not enough to make a significant difference in shot placement.
     
  15. Neil Winston

    Neil Winston Well-Known Member

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    As a multi-year combatant in the Perazzi rib wars with my own MX-2000, I use the comb to make it shoot where I want. In my own case, adding rib adjustments did not make it twice as confusing, it made it four times as confusing. Or maybe eight.

    The problem, Rastoff, though you may be completely right in your analysis, is that you have to start with knowing you want 70% high and how do you get there? In other words, with a new gun which weighs differently from the old, has a different trigger, and everything else is unfamiliar, why pick any number over another? And once you shoot it and jack the comb around so you can hit something, why go any farther? OK, if you run out of adjustment I can see going after the rib, but short of that, why taunt the bear?

    Neil
     
  16. Mark425

    Mark425 TS Member

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    Smok'n Joe,

    All they are is DIY custom made guns. The idea is to set the POI/POA and sight picture you want and leave it alone. Its that simple.

    As you pointed out, those that dont.....dont break birds. Same type of folks buy a new golf club every time they have a bad day....they dont play golf very well either.
     
  17. AveragEd

    AveragEd Well-Known Member

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    I wouldn't own a non-adjustable trap gun unless I just stumbled upon one that shoots well for me without those features. And I wouldn't buy a new trap gun that didn't have choke tubes unless the way the fixed choke barrel was bored had something to do with the gun's appeal. But I treat all those "adjustabilities" the same - I find which settings and which choke work best for me and then leave everything alone.

    Joe, if you take the time to figure out how those adjustments affect the gun and then give any changes you make time to prove or disprove themselves to you, you'll find the magic combination and hide the wrenches from yourself.

    One way to get a new adjustable gun in the ballpark is to pattern the old non-adjustable one that shoots well for you and then adjust the new one until it shoots to the same point. From there, it should be just fine-tuning.

    Ed
     
  18. Rastoff

    Rastoff Active Member

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    Neil,

    You are right, but I wasn't talking about what a shooter "needs" so much as "how" to do the adjustment.

    I also agree with what you and Ed are saying about adjusting it to find what works and then LEAVE IT ALONE!! I've watched guys just kill themselves with adjustments. Often I think most would be better off without any adjustments.

    I have owned two guns with adjustable ribs and I don't think it really helps me. In the future, I'll be looking for guns with an adjustable comb only. If I can swing it, I'll have a custom stock made and do away with all the adjustments.

    Doug
     
  19. zzt

    zzt Well-Known Member

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    Mo Bill, There are several versions of the MX-15 manual out there. The earlir version that came with mine was the clearest.

    Here is what you do. Put the rib in the highest front position. Then shoot for POI at about 15 yards. Adjust your comb so that when you sight down the rib at you target, you hit your point of aim. The gun is now set up for 50/50. Record the amount of spacers you have under your comb. That is your starting pack. You will never use less than that amount.

    Once you have done this, follow Perazzi's instructions (at least in my manual). For every notch you lower in the front, add 2mm of spacers under the comb. They are marked, but hard to see. You should have a pair of 1mm, a pair of 2m, 2pair of 4mm and a pair of 8mm. For every notch you raise the front, remove 2mm. That way, your bead alignment is always the same, and you raise or lower POI by 3" @ 40 yards for each notch.

    Example: for the sake of argument, let's say that with the rib in the highest front notch, you hit POA with a 4mm spacer on each post under the comb. Fine. Now remember that 4mm. Let's say you think you want your gun to shoot 80/20 or 9" high at 40 yards. Move the front of the rib down 3 notches (and move the rear up 3 notches), then put 6mm of spacers on each post (one 2mm spacer for each notch moved) for a total of 10mm on each post. Your gun is now shooting 80/20 and you have the same bead alignment as you did when it was 50/50.

    Now let's say 80/20 is a tad too high for you. So you raise the rib one notch in front (and lower one notch in the rear) and remove 2mm of spacer from each post. Now you are at 70/30, or 6" high at 40 yards.

    Some of the posters above are wrong. When you move the rib only, you change POI. When you move only the comb, you change POI. When you move both together, the changes are additive. About 65% of the change come from the 2mm adjustment to the comb. The remaining 35% is from adjusting the rib. So if you don't care about having the same bead alignment, you can fine tune your POI using only the rib or only the comb.

    Moving only the rib down by one notch, and leaving the comb alone raises your POI by about an inch at 40 yards. Leaving the rib alone and adding 1mm of spacers under the comb does the same. 2mm raises POI about 2". So you can see how Perazzi got their formula. Lowering the rib one notch in the front raises POI by about an inch, and adding 2mm of spacers to the comb raises POI by about 2". Adding, that comes to the 3" Perazzi says when you do both. Remember, the one notch/2mm rule is only if you want to see the same bead alignment. There is no rule that says you can't adjust only one or the other.
     
  20. jimrich60

    jimrich60 Member

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    zzt

    That works because the Perazzi rib pivots in the middle, allowing the rear of the rib to rise if the front is lowered (and vice versa). However, most adjustable ribs pivot at the rear, so moving the front up or down does not move the rear of the rib. Thus you cannot raise the comb and use the rib to "regain" a front/mid bead alignment without changing the POI that you just set with the comb. Even with the Perazzi, as your explanation notes, using the comb and rib creates an "additive" change (or one could subtract from the other). It just allows one to use the two parts (rib and comb) in combination to change POI and keep the "desired" bead alignment. But again, this only works for a mid pivot rib.

    Jim R
     
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