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How to calculate slope of add on rib

Discussion in 'Shooting Related Threads' started by perazziboy1, Oct 19, 2012.

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  1. perazziboy1

    perazziboy1 TS Member

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    Hi, i am trying to make an add on rib. I need to figure the slope of the rib to get my POI where i want it with stacked beads. right now i have alot of space and would like to have a true figure 8 sight picture. I suck at trig so is there anybody that can tell me how to do the measurements so i dont have to ruin alot of stock? Thanks Steve
     
  2. Cooter

    Cooter Member

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    What I did was go to a hobby shop and got a piece of 1/4 x 1/4 balsa wood and make a add on rib out of it. then go to the pattern board and shoot and adjust till you get it where you want it.Send measurements to Keen or rib maker.
     
  3. acss

    acss Well-Known Member Supporting Vendor

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    if gun shoots with correct poi as it is w/ beads spread--then just have your center bead moved forward! - cooter is right tho using a piece of wood works
     
  4. hmb

    hmb Well-Known Member

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    The POI is determined by the height of the front sight. Now add the height of the comb to the equation. Now take the balsa wood add on rib and place it between those two points. Move the rear of the rib up and down until you get the right slope. HMB
     
  5. federal paper

    federal paper TS Member

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    PM Dave Berlet he builds add on ribs and can tell you what a certain measurement in fall on how many inches in barrel length etc . There is an equasion for this'he knows it and I am sure he would be glad to help you.
     
  6. federal paper

    federal paper TS Member

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    PM Dave Berlet he builds add on ribs and can tell you what a certain measurement in fall on how many inches in barrel length etc . There is an equasion for this'he knows it and I am sure he would be glad to help you.
     
  7. shotgunpeople

    shotgunpeople Active Member

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    I adjusted my stock until I was crushing targets using just straight away targets. I had a huge spread between the beads that was distracting to me after the adjustment.

    I added a wooden block to the rear of the gun, whittled it down so I had the front bead just resting on the block...Measured the height of the block then took a piece of plastic, made it the same width and height, and glued it on the rib...I was going to knotch out the block and insert a bead to represent the center bead but found that un-necessary.

    Now when I line up, I only see the front bead resting on the block and it looks like I am looking down a full rib. Sorry I didn't get the camera lined up so the bead is in the center in the second picture...

    Works well for me.

    Dave in SC
    shotgunpeople_2008_030393.jpg
     
  8. perazziboy1

    perazziboy1 TS Member

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    Thanks for all the replies. Good info here as usual! I know there is a formula for calculating the correct slope, and I know I saw it on here before. I will do a search and try to find it again.
     
  9. mikkeeh

    mikkeeh Active Member

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    shotgunpeople: Makes perfect sense. If you then measure the height of the block, you can determine how much taper you need. Dave Berlet did a rib for me some years back. He did a similar type measurement using short pieces of derlin rib at the rear of the barrel. Good luck!
     
  10. shotgunpeople

    shotgunpeople Active Member

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    TjayE,

    I hope I explained the PM good enough..sometimes I can't explain in the right words...

    Let me know how you make out..People who try my gun can't believe how it makes the appearence of a full rib...No added weight either.

    Although I don't shoot long yardage, the last ATA Handicap event I shot, I had the last 75 straight, and 25 in shootoff with this setup.

    Dave Berlett is probably the best in the business. I would rely on him for any add on rib !

    Dave
     
  11. perazziboy1

    perazziboy1 TS Member

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    I have contacted Dave about getting ribs built for my gun. I do have access to delrin and mills, wanted to try that out first. Thanks again for the PMs and great responses! Steve
     
  12. perazziboy1

    perazziboy1 TS Member

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  13. Dr.Longshot

    Dr.Longshot Banned Banned

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    I BELEIEVE IT IS FOR EVERY .030= 2 INCHES OF POI OVER THE 30 INCH BBL.
    .060= 4 INCHES Etc
    GB
    DLS
     
  14. whiz white

    whiz white Strong Supporter of Trapshooting Banned

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    Slope is defined as "rise over run."

    Calculated, it is the height divided by the length, which is a number, less that or equal to 1, not an angle. You can get numbers greater than 1, but not doing this small of an angle. One can then find the "arctan," which means find the tangent angle of that above calculation. You can go to Google and find the decimal-to-angle result.

    There are six trig functions (well, three, and their reciprocals), and by definition they are: sine (y-distance divided by the hypotenuse {sometimes called "r" for radius of a unit circle}), cosine (x-distance divided by the hypotenuse, and tangent (y-distance divided by the x-distance), or sine divided by the cosine. Trigonometrically speaking it appears to be complicated, but mathematically, it's pretty simple.

    WW
     
  15. zzt

    zzt Well-Known Member

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    Steve, if you really want the folmulae PM me and I'll give it to you, plus an explanation on how it works. You got some good hints here and some really, really bad info. Here is a simple way to accomplish what you want without resorting to much math.

    You currently have your POI just where you want it, but cannot stand the sight picture. You want a figure 8. Well, that is subjective, so I 'll give you another way to go about it. Place a quarter on the rear end of the rib (tape it if necessary) and shoulder the gun. I f you can just barely see the base of the front bead over the quarter, you are effectively sighting flat down the rib. If not, add another quarter, or nickel or dime so that the top of the coin stack just allows you to see the bottom of the front bead.

    If the rear of the rib were as high as the coins, you would be seeing what is called stacked beads. That is, the bottom of the center bead is in line with the bottom of the front bead so they are superimposed. Since the front bead is larger, you see the rear bead and the top of the front bead. They are "stacked" but many call this a figure 8. A true figure 8 would require a slightly lower rear rib, so remove one dime, measure the height of the remaining coins and make your new rear rib height that much higher than your old, while keeping the front height the same.
     
  16. Setterman

    Setterman Well-Known Member

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    zzt is on the right track. I use 1/16" stock shims and tape them to rear of the rib. When I can only see the front bead and no rib, I'm there....or very close.

    Dave Berlet has done 2 for me.

    Caution: with an add on rib, most require 3/16" of material at the muzzel. Add another 1/4" to 1/2" in rib slope, and you'll need some very long adjustable comb posts.
     
  17. Dennis DeVault

    Dennis DeVault Well-Known Member

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    There is a formula for calculating the height that a rib will shoot it is the number 1080 x the amount of slope in the rib and divide that number by the barrel length and that will give the approximate POI here is and example:
    1080 x 3/8" of slope 3/8 is .375 = 405 divide that by the barrel length lets say 32" and the answer is 12.650 inches of height. A shorter barrel will be higher and a longer barrel will be lower. Another way to ball park is for every 1/8" of slope the POI changes roughly 5". Hope that this will help.

    Dennis DeVault
     
  18. perazziboy1

    perazziboy1 TS Member

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    Thanks for all the great advice and thanks Dennis for the formula. Steve
     
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