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need POI percentages

Discussion in 'Shooting Related Threads' started by grunt, Jun 8, 2012.

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  1. grunt

    grunt TS Supporters TS Supporters

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    Need info on 13 yard poi. 60/40 1 inch ect. Thanks
     
  2. Shawn

    Shawn Member

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    You have it
    70/30 2 inch

    80/20 3 inch

    90/10 4 inch

    100% 5 inch

    This is assuming a low rib top Single.

    Shawn
     
  3. mike campbell

    mike campbell Active Member

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    Just multiply the shift at 13 yds times 3 to get the shift at 39 (40) yds. Shown is the nominal "30 inch pattern @ 40 yds"


    POIpic2-1-1.jpg
     
  4. Gapper

    Gapper TS Member

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    I see why the U.S.A ranks something like 25th in math, as compared to other countries.
     
  5. mike campbell

    mike campbell Active Member

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    OK, educate us. What math errors are you referring to?
     
  6. grunt

    grunt TS Supporters TS Supporters

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    Thank You.
     
  7. Garry

    Garry Active Member

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    Mike,

    6 inches high at 40 yards would put 21 inches above and 9 inches below for a 70/30 POI.

    12 inches high at 40 yards would put 27 inches above and 3 inches below for a 90/10 POI.
     
  8. Martinpicker

    Martinpicker Active Member

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    Wait a minute! Forget inches! If I pattern a gun at 13 yards...stationary target, shot from a rest,...I draw a horizontal line through the target before I shoot. I shoot like this twice. Then I count the number of pellet holes above the line vs. those below the line...I get 70% of the total pellets above the line and 30% of the total pellets below the line!!!.....isn't my gun shooting 70/30 ?????? Jack
     
  9. JACK

    JACK Well-Known Member Supporting Vendor

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    I pattern all guns that I likely will shoot. About 6 shots from 13 yards. I patterned one the other day that shot 5" high (about right for me) and 3 shots right of center and 3 shots centered. I do it off hand, and usually I get 3 shots centered and 3 shots left of center... Hmmmm
     
  10. mike campbell

    mike campbell Active Member

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    "6 inches high at 40 yards would put 21 inches above and 9 inches below for a 70/30 POI."

    Garry,

    21 inches is the fraction of the circle's diameter above the target. And you're correct, the ratio of the two portions of the diameter is 70:30. But that ratio doesn't have any meaning to me.

    When I use the term 70:30, I mean to say 70% of the area of my circular pattern (theoretically containing 70% of the pellets that are in that circular pattern) is above and 30% is below. Since I'm interested in the fraction of the pattern AREA above and below the target, I need to find the area of the slice, known as a "segment", that lies below the target so I can ratio the two. When I do that, this is what I find...



    POIpic4.jpg


    The numbers I used are for a 30" circle, but of course the same ratio would hold true for any size pattern at any distance.

    I'm still waiting to see Gapper's math.
     
  11. Gapper

    Gapper TS Member

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    Mike,

    Your calculations are correct if we are looking for shift in percentage area of a flat surface. We're looking for a way to relate Point-of-Impact shift. - P.O.I. - As in a single point. The single center point of a standard 30" pattern @40 yards.

    Easy to see how this can be misleading. It's OK to use inches though, and for many it would be easier.

    Using this square inch area stuff only leads to confusion and there is no way to convey a point of impact being more than 100% high. It's also not a totally accurate representation of the shot event, so when we use the sq. in. for something that should be rightly measured differently, the calculations are wrong.

    Everybody understands what goes on here.

    A flat surface "area" representation of the shot cluster, would be one way to measure the pattern "area shift", but it is not technically correct. And we aren't measuring "area shift". If we try to measure the "area" of a something roughly spherical then the calculations come out wrong.

    We have to get out of the "plane geometry" area into something a little more advanced, if we want to be "technically correct". The shot event is multi-dimensional, and the flat suface model doesn't work when we use more accurate measuring systems, because it is not a complete representation of the event.

    The percentages will change MUCH even when using solid geometry (Volume of an -irregular- spherical object) which is a more accurate, but much more difficult to measure. We do want to be super-duper accurate, right? To show-off our high level of intellegence. So nobody uses the area thing cause no one likes getting laughed at.

    Using P.O.I. shift (the center point) works with different ways of measuring shot charge shift, and, as an added bonus - it's easy to figure out in your head. Ten percent of 30" equal 3"; therefore 3" equal 10% shift in height. So Easy.

    Please, don't start on counting individual pellet impacts.

    The old way (K.I.S.S.) usually works pretty well. A 30 inch pattern, how far above or below a single center point. Then everyone understands what you're saying. Even though true, no one would relate patterning their gun on a 706.8583471 sq. in. plate.

    I like the graphics, and I do understand what your calculations convey. The math ranking quip was not solely directed toward you. No doubt my sense of humor went a bit over some heads. Regards, GAP
     
  12. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    No wonder Einstein didn't trapshoot.
     
  13. Gapper

    Gapper TS Member

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    Einstein was indeed a skeet shooter. His Browning Superposed shot something like 353.4/353.4 sq. in., but he always called it 50/50!
     
  14. BigM-Perazzi

    BigM-Perazzi Well-Known Member

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    I know, that's why I said trap. In skeet he didn't have to worry about POI...
     
  15. mike campbell

    mike campbell Active Member

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    Gapper,

    I can see there's a huge disconnect in the way you and I are wired, so no point in arguing the many points you raised. Apparently, unlike you (and everyone?)I AM looking for a percentage area shift, not a point shift. If I were shooting a single slug I could see how a "point" shift would be useful. But I'm shooting a swarm of several hundred pellets and, if I'm going to make any sense at all of the swarm's height in relation to the target, my little pea brain has to simplify that to a planar representation. Therefore, to me, a POI shift in inches is totally useless without the accompanying image of a target in a circle. I tried to help with a simple visual aid, which is exactly what I picture in my head when the discussion of POI comes up. It wasn't my intention to showcase my math skills until they were challenged.

    I don't know who the "we" is you're referring to (can't be the royal We, unless you're Neil Winston), but since "nobody" uses the area thing, I guess I'm off the hook for any future attempts at helping people visualize their POI shift. That's a great relief to me since all my guns shoot exactly where I look. For your camp that would be 0" high @ 40 yards. Thanks for sharing a good laugh.
     
  16. Gapper

    Gapper TS Member

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    Mike,

    I think I may have identified the flaw in your (logic) calculations. In your response to Garry the graphic shows a portion of the circular pattern "cut off", correctly identified as a "segment" of a circle. This plane geometric formula won't apply correctly here.

    Because the circular patterns dealt with here are fixed and cannot properly be "cut off", consideration can only be given to actual pattern shift of the entire 30" pattern circle as a whole, in relation to it's own former postion.

    The first graphic is a better represention of this action, and clearly shows that the total independent areas of respective pattern circles are fixed in relation to the other. The amount of area displacement would be a constant, depending solely on the distance the pattern circle moves. A one inch movement would be on the Y axis and area displacement would remain constant for each inch of movement. This coincides with the old 3" of 30" = 10%.

    I know calculus can be used to find the actual square inch displacement for each interval, but math is a drag anyway. If you can point me to the formula I'd appreciate.

    Thanks for joggin' my noggin. GAP

    P.S. In trying to find an easier way to explain, each shot fired is a singular occurence, so the 30" pattern is always evaluated @ 100% - no way to "cut-off" a sector or section. The only true measurement is bias or shift of the 100%.
     
  17. open choke

    open choke TS Member

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    With the inch method I get one number and if you do the pellet count I get another which one is right?

    Or do I say my POI is ___ inches high and my pattern spread is ___/___% spread above and below the POI?

    Example when I did my count using a 410 pellet 1oz shell shit 2 times I have 820 pellets I had around 35 below my aim crosshair. So I counted the less amount did the math and got 94.75% but my friend says its 70/30 using the inches above crosshairs. So who is right?
     
  18. Gapper

    Gapper TS Member

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    Pellet count won't work because no two patterns are alike, the core of pattern (the amount of pellet strikes) is always more densely populated because of the shape of the shot charge leaving the barrel and effects upon it. This would skew any calculation.

    Percentage works best properly as a percentage measure of linear movement of the known 30" circle.

    Anything else adds confusion. GAP
     
  19. open choke

    open choke TS Member

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    Well I used a 30 inch circle. Does this change anything?
     
  20. Gapper

    Gapper TS Member

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    The 30 inch circle is proper, as it is the industry standard, for measuring POI shift at 40 yards, as well as choke/load selection. Skeet loads and .410 loads are usually evaluated at shorter range, 25? yards, because they are more specialized than 12 gauge trap/field. GAP
     
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