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calculate POI change w/ add-on rib?

Discussion in 'Uncategorized Threads' started by skeet_man, Jan 2, 2008.

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  1. skeet_man

    skeet_man Well-Known Member

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    Long question coming up:

    I'm planning on adding a line of delrin add-on ribs to my business (bbl weights). I was originally going to only offer parallel ribs (no poi change) cut to the exact width of the existing rib (any width), but thinking about it further, I'd also be able to make ribs to change POI as long as there is a thickness of material available which would already match the width of the rib (in other words, 5/16" thick stock would work for a Remington 1100 rib which is just over .300 wide). I'd be able to make ribs to adjust POI in the following widths: | 1/8" | 1/4" | 3/8" | 1/2" | 5/8" |.


    How do I determine how much difference there needs to be between the front of the back to adjust the POI? I know I will need to know current POI, desired POI, barrel length, and shot distance, but where do I go from there? For example, say someone has a 34" barrel that shoots 60/40 now, and they want to make it shoot 90/10 with an add-on rib. How would I go about figuring out how high to make the breech end of the add-on?
     
  2. chipking

    chipking TS Member

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    simple math (as an example) RL (rib length front to rear) 34" AC (amount of change in rib height front to rear .1" R range in inches 35 yards = 1260

    NOW R devided by RL 1260 /34 = 37.06 X AC .1 = 3.71" of change at 35 yards

    If you are using POI percentages based on a 30" pattern every 3 inches is a 10% POI change.
    All of this assumes that you are looking down the rib add on the same way you looked down the original rib.
    --- Chip King ---
     
  3. zzt

    zzt Well-Known Member

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    The math isn't as complicated as all that. Also, you want your clients to specify the change they want in POI in inches at a set distance. Don't fall into any of this 80/20 crap, because you will get a different interpretation from everyone you ask.

    Here's the simple math. You don't even have to use trig. I shoot a 35" barrel. My eye is 45" behind the bead when I mount the gun. If I raise the comb by 1/8" (.125"), that also raises the POI by 1/8" 45" in front of the bead. So for every additional 45" in front of the bead, you raise POI by another 1/8". Move 8 45" in front of the bead and you have adjusted POI by 1" at 8x45"=360". Convert to yards 360"/36"=10 yards.

    So, if I raise my comb by 1/8", it raises my POI by 1" every 10 yards. That works out in practice as well as in theory. So your problem is to find out how much to taper the rib so the bead alignment stays the same. Easy. The rear of the flat portion of my rib is 29" behind the bead. The new height needed at the rear of the rib is proportional to the height of my eye on the comb. So I raised my comb/eye by .125" (1/8"), so the height of the rear of the new rib needs to be 29/45=.644 x 29" = .0806. (edited after the fact for clarity)

    So if I came to you and said I want to raise my POI by 8" @ 40 yards, you would use the measurement I just gave you and tell me that I would have to raise my comb by 1/4", and you would make me a rib that tapered from 0" at the bead to .161" at the end of the flat section.
     
  4. skeet_man

    skeet_man Well-Known Member

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    Ok, had to read that a couple times, but I think I understand now.

    I'll use my gun for example (with slightly more complicated #s). It measures 41" from my eye to the muzzle, 24" of add-on rib would be what I want (rem 1100). 24/41=.585. So if I wanted to raise the POI 5" @ 40 yds I would add 0.142 on the comb height (360/41=8.78, 1/8.78=.114 at the comb would equal 1" @ 10 yds, 4" @ 40 yds). Therefore, the add-on rib I would need would be .083" in the back tapering to nothing @ the muzzle (theoretically, since you wouldn't be able to make a rib any thinner than probably 1/8" at the muzzle to accommodate the front bead threads, so the real rib would measure .208 @ the back and .125 @ the front).
     
  5. Rollin Oswald

    Rollin Oswald Active Member

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    Ian,

    Be darn sure you understand it and can compute it correctly before offering ribs to change POI.

    Probably unnecessary to mention but higher ribs often require a higher comb. My guess is that not all those who will be interested in add-on ribs will realize their relationship with comb height.

    Rollin
     
  6. skeet_man

    skeet_man Well-Known Member

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    rollin- that was my intention by posting here. I know i can do the parallel ribs easily, but had an epiphany this morning when I realized I could change the way I draw the ribs in CAD to allow them to be made to adjust POI so long as they are made from a std width of material. I just didn't know what calculations were necessary, and it appears that zzt gave me the information necessary to do that.

    BTW, I believe I solved the adhesion issue, so parallel ribs should be available shortly. Still need to get material to the cutter so they can run tests on it to make sure it'll cut OK, but they can do anything up to 1/4" right now. Will hopefully have info on my website in the next couple weeks.

    Also, does the 1/8" at the comb translating to 4" at 40 yards hold true based on your knowledge? Trying to get as many opinions on this as possible so I can hit the ground running when I get started.
     
  7. tandemdude

    tandemdude TS Member

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    If you don't know how to figure the POI change out, do you really need to be building ribs? So far I have read that you don't know how to attatch the rib, and you don't know how to build it to make it change POI, but you are going to start offering them on your web site. Good Luck with that, Norm
     
  8. zzt

    zzt Well-Known Member

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    Skeet, don't get hung up on the 10 yard thing. I use it because it is convenient and the math works out for my gun.

    You want to raise POI by 5" at 40 yards. You know your bead to eye distance is 41" There are 1440" in 40 yards. Divide that by 41" and you get 35.122 bead-to-eye units in 40 yards. Divide the 5" at 40 yards you want to raise POI by the number of bead-eye units and you get 5"/35.122=.1424" That's how much you have to add to the comb. Figure out how much you have to add to the rib using the info I gave you above.

    If you do not want to taper the rib to zero at the bead end, .050 thickness will be sufficient. So just add .050" to the rib and the comb and you have the same relationship.

    Let's use your gun and assume the flat part of the rib ends 28" from the bead. So you need a rib that tapers from zero to 28/41 x .1424 = .0972". Common hardware store spacer rings come in 1/16" thickness, so that is what you are going to buy to raise your comb. Three spacers for 3/16" (.1875) is .0903 higher than what you want. So add .09" thickness to the rib to match, and the rib tapers from .09" to .19". That rib and two sets of three 3/16" spacers to raise the comb will do the trick.

    Since you have CAD, you could simply plot everything out life size and simply measure what you want.
     
  9. Setterman

    Setterman Well-Known Member

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    You can also lay it out on CAD when you know the barrel length, and "bead to eye" dimension, and distance to impact. Show your client what your planning to do.
    Rollin is right. Tell them that their stock won't work. Few people realise it. They will need an adjustible comb onger posts. When I did my Super X, I went from 3 shims to 13 shims to make it shoot 100% high. I knew it would be required once I found out the thickness of the new rib at the bead end of the barrel, which can be as much as 1/4" (4 shims) plus the raised taper (another 6 shims) required to get 100% POI at 40 yards.
     
  10. skeet_man

    skeet_man Well-Known Member

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    tandemdude- thanks a bunch for the useful post. Do you think that everyone is born with the knowledge to do everything in life? No they aren't, they must learn new things if they wish to prosper, which is what i'm in the process of doing. I've only been working with this idea for a couple days now, and am ironing out the issues as they come up (of which there have been more than a few). Do you think that people who build gun stuff got it right the 1st time? No, probably not, they most likely learned through trial and error, a step which i'm trying to skip by asking the people on this message board who know more about this stuff than I do. I never jump into anything without doing alot of research first, to do otherwise would be foolish. I'm a skeet shooter by trade, so a POI changing rib is something very foreign to someone with my shooting background, although it is very common in the trap world, hence the reason i asked for advise on a TRAP forum.
     
  11. XT Bill

    XT Bill TS Member

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    I think you're on the right track !

    My BT-99 shoots flat, so I raise the comb, and end up with a bunch of space between the beads, which I would really rather not have, as I have to guesstimate whether or not it looks right when I mount the gun.

    If this space could be eliminated (stack the beads right on each other), or even reduced to something verifyable like one front bead worth of space, that would be sweet !

    For some reason, I've just never taken the time to mess around with cutting a piece of wood on a taper to accomplish this.

    Now, if you could put some sort of adjustable mid bead for fine tuning...

    Hmmm...

    Build it, they will come !
     
  12. Dickgshot

    Dickgshot Well-Known Member

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    Skeet man: I want to suggest that for what you're planning to sell your ribs for, you're getting into a lot of complicated issues. Why do you have to deal with how high a gun will shoot? All you're going to have is a lot of p.o.' ed
    customers if they perceive that the gun isn't shooting exactly where they think it should.

    If someone wants a rib that tapers 1/8" let him order it. I think it will be all you can do to get the dimensions right without having to figure out where the gun will shoot for someone.

    I also want to mention that the transparent double stick tape that I found at WalMarts was very thin, and had terrific adhesion qualities, besides being a lot cheaper than the tape you're contemplating. I would look into it.

    Good luck, and I'll be your first customer.
     
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