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Formula for poi/different barrels

Discussion in 'Shooting Related Threads' started by turmite, Feb 9, 2009.

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  1. turmite

    turmite Member

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    I didn't want to hijack Jolly's thread about the derlin rib he made, but a statement in that thread really caught my attention. The question was "what is the poi"

    How do you determine poi on a add on rib without shooting the gun? Is there a formula that gives poi for different height ribs, on different length barrels with and without adjustable cheeks also being in the equation?

    The reason I ask, I had an aluminum rib made for my grandson's 1100 and have yet to put it on. I am curious if I can determine the poi before he actually shoots the gun with the new rib on?

    Mike
     
  2. KEYBEAR

    KEYBEAR Active Member

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    Call Vicki at Keen sight . When I had my add-on rib made I ask for them to add 10% and that is what I got .
     
  3. zzt

    zzt Well-Known Member

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    Mike, if you raise the comb to match the new rib height, the POI will be the same.

    If you had a vertically tapered rib made, increase the height of the comb by the amount the bead was raised.

    If you bought the rib because the comb was already high and your grandson saw a lot of rib or barrel, you will lower the POI when you attach the rib. You can easily calculate the amount of change. Pick a target distance. Measure the distance between the bead and your grandson's eye while in firing position. Divide the bead/eye distance into target distance and multiply by the amount you raised the bead.

    Example: there are 45" between my eye and the bead. There are 1440" in 40 yards. 1440 / 45 = 32. If I raise the bead by 1/8" (.125") I will lower my POI by 32 * .125" = 4" @ 40 yards.
     
  4. hmb

    hmb Well-Known Member

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    If the top of the add on rib and the top of existing rib are parallel there will be no change in POI with the same sight picture. If the front(muzzle end) of the add on is lower than the rear the POI will be higher. And if the add on is higher in the front than the rear the POI will be lower. HMB
     
  5. zzt

    zzt Well-Known Member

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    hmb, you neglected to say you had to raise the comb to get that "same" sight picture.
     
  6. turmite

    turmite Member

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    Hi guys,

    No, you are not getting what I am after, or I am not explaining it clear enough.

    According to the question on Jolly's thread, the writer ask "what is the poi"

    My question is this. Is there a formula that gives you poi without shooting the gun? I mean, how do you know what the poi is going to be unless you shoot it.

    I understand that if the rib is tapered more narrow from back to front the poi is going to be higher......but I want to know if there is a way to figure the poi via a formula. The gentleman that ask the question got me to thinking about this and I am curious.

    As to calling Keen.....I am not thinking they would be too interested in giving out trade secrets.

    Mike
     
  7. Jawhawker

    Jawhawker TS Member

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    Mike, did you now know the POI of the gun before adding the rib?
     
  8. Shooter R

    Shooter R Active Member

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    I'm somewhat convinced the P.O.I., measured in a pecentage (70/30), or inches (9" high), is nice to know but is less important than how you break the targets with your style of shooting. If you hold a high gun hold and just move the barrel under the target and shoot (with no vertical motion) you need a higher P.O.I. than a shooter who holds on the house, calls for the target, then quickly follows the path, overtakes, and shoots at the leading edge.

    Phil Kiner's article in the latest Trap and Field pretty well describes the advantage of knowing "what's too low, and what's too high". It's practical knowledge tht will help you break targets at a particular yardage. It's a little work, and you need to tie up a trap at your facility for and hour or so while you screw around with the rib/comb, (start on post three, then check (and possibly get ready to comprimise with a lower setting) on 1 and 5.

    In responce to your question: How do you you know the P.O.I. without shooting the gun?, you can use the above formulas, but without YOU shooting it at targets with your style, you'll not know what is best for the yardage you want to set it up for. Good Luck, and I hope this helps.
     
  9. turmite

    turmite Member

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    pheasantmaster I have not added the rib yet, and we had an idea last year, (he was in the 6th grade)that the poi was about 80% high at 35 yds.

    My having not mounted the rib yet is part of the reason I posted this question.

    I suppose I should start from a clean slate.

    I am involved with the AYSSP program in Arkansas, the largest youth shooting sports program in the US. We do not have the latest numbers yet, but estimates are for 4500-5000 young shooters. We are from a smaller school district and have had a increase from 35 youth last year to almost 70 as of right now, and they are still signing up.

    Long story short, I had the honor to meet and talk with Bill Roy, the new Director of USA Shooting in Colorado two week ago at a coach's clinic. It looks like due to the unbelievable growth and success of the Arkansas program that we may get one of the regionals for the USA Shooting tryouts. I ask him how they wanted the guns poi. His answer surprised me. 100% high at 40 yrds!

    So, since I have not mounted the on the gun, and had it built to just add to the top of an existing rib, I know I am going to have to change it to get that poi and therefor the request for the formula if there was in fact a formula! I am going to have to make some changes to this rib before mounting it and would like to be close so that I only have to do it once.

    Mike
     
  10. zzt

    zzt Well-Known Member

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    Mike, you simply cannot find out the POI of a gun unless you shoot it. If you give the same gun, set up the same way, to five different shooters, each will likely have a different POI. It depends on your facial structure, your body type, and how you mount and hold the gun. Once you know the POI for a given shooter, you can easily calculate what will happen if you do "x", but not before.
     
  11. Jawhawker

    Jawhawker TS Member

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    turmite, a mathmatical formula could be used if you really new what the POI is now before adding the rib but it would still be an approximate. It sounds as though you haven't patterned the barrel in stock mode so I would suggest for the sake of your shooter to go and do it. As zzt stated, the boy should be the one to do this if he is going to be the user. This gives you a break via just overseeing the situation.
     
  12. turmite

    turmite Member

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    zzt, pheasantmaster and others,

    Thanks for the input. You have pretty much confirmed what I thought, but wanted some other more experienced input.

    p/m he competed with the original set up last season and according to the way we patterned the gun, at 35 yrds it was about 80% of a 30" circle above the poa.

    I was hoping someone could say take a 1/8" total off the front end of the new rib and you will be at 100%.

    I will post a pic of the rig when I get it finished.

    Mike
     
  13. Dr.Longshot

    Dr.Longshot Banned Banned

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    I was told that raising the rear of the rib and leaving the front at the same location, adds 4" inches to the POI over 34 inch Bbl, so I would take that to mean for every .030 you raised the rear of the rib, you gain 4" inches of POI
    Let's use this as an example lets say you have a tapered rib made and it is
    .500 thousandths or 1/2 inch, and the slope over-all on the rib at the muzzle
    .470 thousandths and keeping the same bead alignment you would realize 4" inch
    rise in POI.

    Now lets go back again on same .500 thousandths at rear and .440 thousanths at the muzzle and same bead alignment the POI would be 8 Inches higher.

    Now .500 thousandths at the back and slopes down to .400 thousandths in front the POI would be 12 inches higher, this would be almost a 100% high shooter.

    This dimension was given to me by Jimmy Ljutic, he was very close on it, as we did not fire my gun just sent him the Bbl and had him cut down the pickets and leaving the rib the same at the reciever, my Mid-Rib Ljutic was shooting 80/20
    and I had him raise the impact to 110% and he lowered the front of the rib by
    about .090 thousandths

    Hope this helps.

    Gary Bryant
    Dr.longshot
     
  14. ouch

    ouch Member

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    Turmite, The ribs he made are for me. I am a simple person with poor math skills. So I bought a 5/16" wide piece of balsa wood and taped it to my existing rib.Then used a white headed stick pin to represent the front site and a sewing style strait pin for a mid bead.

    Then played with my barrel laser tapering the balsa til I got the point of impact desired using the Neil Winston method. Then went to the pattern board and shot my reloads. Still fine tuning (tapering ) the balsa. Then started shooting birds from post 3 with the machine locked on strait from 16 yrds and 22 yrds.Then just shooting regular rounds.

    I also adjusted my comb during this process to give me same site picture as the comb was tapered. My results were the 3 ribs Josh made, a 16 yard rib,22 yrd. rib. And an 1/8 " taper for my 1100 for sporting clays.The 4th rib,the shortest one will replace the sites on a Remington .22 pump that I will use to teach new shooters instinctive shooting.

    Next I will build up an old stock with Bondo or Kneedalite sp? and shape it down by the same drill.When I am happy with it get it duplicated and finish it out. Hope this helps. Richard
     
  15. turmite

    turmite Member

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    Dr Longshot and ouch.....both of you have helped. Dr the numbers you gave is something I can now plot in my can and apply the same formula for my grandson's 28" Remington barrel. Before I actually cut the rib again, I think I will use the balsa. As far as that goes I can actually cover the balsa with a couple of layers of carbon fiber when done with it and let him use it a couple of times before going to the aluminum. Thank you both!

    Mike
     
  16. Neil Winston

    Neil Winston Well-Known Member

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    turmite, I wonder why you are trying to get 15 inches high. Is the shooter shooting under the birds?

    I'm glad you have had your grandson shoot for POI. I do, wonder, though, what's making the POI so high now. Is he seeing a lot of rib? if not, this is a _very_ unusual 1100.

    Assuming you have a comb that can be made to rise enough to give the same sight picture, you just need a rib (roughly speaking) about 1/40 inch lower at the end for every inch you want to raise the POI. Since here it's maybe 5 inches, about 0.12 inch should do it.

    Neil
     
  17. zzt

    zzt Well-Known Member

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    Mike, you can easily figure out the answer to your last question by using the formula I gave you above. You just have to solve for a different factor.

    The gun now shoots 12" high at 35 yards (about 80% of a 30" circle high). You want 100%, so you have to move POI 3" upward at 35 yards. 35 yards x 36" = 1260". Divide that by your grandson's eye/bead distance. In my case that is 45". So 1260 / 45 = 28. Divide the distance you want your POI to move by 28. In your case that is 3", so 3" / 28 = .1071" That is how much I would have to lower the front bead to move POI up by 3" at 35 yards, while leaving the comb height alone.

    Say your grandson's bead/eye distance is only 40". Then 1260" / 40" = 31.5" so 3" / 31.5" = .0952". That's just a touch over 3/32".
     
  18. turmite

    turmite Member

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    "turmite, I wonder why you are trying to get 15 inches high. Is the shooter shooting under the birds?"

    Hi Neil,
    I explained a little in one of my earlier posts about meeting Bill Roy of USA Shooting from the Colorado Springs training center. My grandson wants to give the Olympic Trap a try and I questioned Mr. Roy about the poi of the guns they shoot at the Center. He is the one who told me the guns were set up to shoot 100% high at 40 yds. My grandson ended the season last year breaking 23's and a couple 24's, but never made the 25 straight.

    "I'm glad you have had your grandson shoot for POI. I do, wonder, though, what's making the POI so high now. Is he seeing a lot of rib? if not, this is a _very_ unusual 1100."

    I am a stock maker by trade, but have made very few shotgun stocks....I hope that changes very soon! When my grandson started shooting trap, he had no experience with a shotgun, and in fact was afraid to shoot one. It became my mission to make him the best stock I could that would reduce recoil, and allow him to compete. Remember, he was only a 6th grader. He had shot 22's and air rifles but had never fired a shotgun. Now with that said, I had absolutly no experience with trap shooting so I got on the net and began to research the "best" way to set up a trap gun. It seemed from my research that most wanted, or suggested a 70-80% high gun. The factory rib that is on his 1100 is a flat shooting rib, so he doesn't use the rib at all. I assumed that if I elevated the rib, so that he could look down the rib, it would get him that one or two other birds he needs. I know about assuming, and that may well be the case here.

    I thank you and zz both for your help. Now please tell me if my assuming is wrong or not? Honest question!

    Mike
     
  19. Neil Winston

    Neil Winston Well-Known Member

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    Sounds fine to me, Mike, except. . .

    you conclusion that "he doesn't use the rib at all" is not exactly what I was after when I asked if he sees a lot of rib. I guess the full answer is that he sees a lot of rib but uses something else to align the shotgun to point where he wants. If we can go about this another way, his eye is well above the breech, right?

    I'd think he'd shoot better if he had a rib to look down and I'm not sure it makes much difference exactly what the the pitch is (use zzt's 1/10 inch for a start). Just something, really anything to give him more visual feedback.

    Can I say "except" again? Are you sure you heard Bill Roy right? I don't shoot bunker but that 15 inches high figure he gave you is not what I would have expected at all.

    Good luck with your project in any case.

    Neil
     
  20. zzt

    zzt Well-Known Member

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    I'll second that Mike. Go back and ask Bill Roy what POI Olympic Trap shooters use. Most that I have heard use anywhere from 50/50 to 70/30. One doubles shooter uses 80/20. 100% high @ 40 is for ATA Trap. About 60/40 is about the average for Olympic Trap.
     
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