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POI Mathmatical Calculation?

Discussion in 'Uncategorized Threads' started by cimmaronkid, Apr 28, 2007.

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  1. cimmaronkid

    cimmaronkid TS Member

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    Ok, I'm a trap shooter and lazy so I am trying to find a fast method of doing this. If you know the POI of your barrel and want to raise it by putting on a raised rib that is tapered in height, is their a formula that you can use to calculate the height of the rib at the muzzle knowing the height at rear and the distance between the two? I know I will have to go to a raised comb, etc, but was just hoping that there was a way to calculate this. Pat
     
  2. Harold

    Harold TS Member

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    Take the difference and divide it by the distance from the front measurement to back measurement then multiply it by the range you will be shooting. Example: 1/8 inches difference per 30 inches barrel length would be a rise of 1/8 inch every 2 1/2 feet, which is 1 inch every (2.5*8=)20 feet, which is 3 inches every 20 yards, or 6 inches every 40 yards.
     
  3. zzt

    zzt Well-Known Member

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    There are 45" between my eye and the front bead when I mount my gun, so a change of 1/8" at bead or comb changes POI by 1" @ 30 yards. Use Harold's formula, but use the distance from your eye to front bead instead of barrel length.
     
  4. cimmaronkid

    cimmaronkid TS Member

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    Thanks guys, I appreciate the help. Pat
     
  5. Harold

    Harold TS Member

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    zzt, check your math. If you raise the comb 1/8 inch and your comb to bead length is 45 inches, that is a slope of 1/8 * 36/45 = 0.1 inches per yard. Multiplied by 30 yards you would get a 3 inch increase in your POI for a 1/8 inch change to the comb.

    Using my method for the same example if you raise your comb 1/8 inch at a distance of 45 inches from the bead, and if you wish to slope the rib to keep the same figure 8 (or whatever) sight picture, the rib will have to be raised 1/8 * 30/45 = 0.083 inches at 30 inches from the bead. This results in a slope of 0.083 inches *36/30 = 0.1 inch per yard, which is 3 inches at 30 yards.

    We get the same answer, but I think my calculation is more useful for the purposes of designing the rib.
     
  6. Rollin Oswald

    Rollin Oswald Active Member

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    Pat,

    Raising the POI could be accomplished more easily by simply raising the comb. The only benefit of a tapered rib would be the ability to get a figure-8 bead alignment. providing your calculations were correct.

    Rollin
     
  7. jbshep

    jbshep Member

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    Rollin is right. The calculations offered are correct, but only if you use the rib as you would a rifle sight; most trapshooters know that doesn't work. The front bead does appear in your periferal vision, but you should be looking at the target, not the bead. Adjusting the comb height is the ONLY way to effectively the angle of your eye to the target and hence, change POI. An adjustable rib is useful in establishing the alignment that you want to see between the front and mid bead to assure consistant mounting; but once you are confident of your mount, you can forget about the rib all together.- Brad
     
  8. zzt

    zzt Well-Known Member

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    Harold is correct. My post should have read 1" @ 30 feet, not 30 yards.
     
  9. phirel

    phirel TS Member

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    The math is fun but might not directly apply to the POI change. The two critical points are actually the height of the eye in relation to the front sight. Raising the comb will raise the height of the eye and for many shooters cause the hear to remain more erect. Raising the come 1/8 inch will with some shooters result in the eye actually becoming elevated something like 3/8 inches more or less because the head is not "rolled" forward and down as much.

    I have long suggested to shooters that the classical figure 8 relationship between the middle bead and the front sight be ignored. I believe this configuration is irrelevant.

    Pat Ireland
     
  10. recurvyarcher

    recurvyarcher Well-Known Member

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    Pat, if the hear is more erect, will I have to change the kind of ear plugs that I use to compensate?
     
  11. phirel

    phirel TS Member

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    Yes- You should use lighter weight ear plugs and balance them with heaver shoe laces.

    Pat Ireland
     
  12. hmb

    hmb Well-Known Member

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    Pat how do you raise the come? HMB
     
  13. phirel

    phirel TS Member

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    I will not answer that question by stating that you turn the screws. I am much too refined to stoop to that level.

    Pat Ireland
     
  14. Rollin Oswald

    Rollin Oswald Active Member

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    Great reply, Pat.

    Rollin
     
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