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poi related to shell speed?????

Discussion in 'Shooting Related Threads' started by yenko, Sep 10, 2009.

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  1. yenko

    yenko TS Member

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    how much does shell speed efect poi? suppose you shoot a 100% high gun with a shell speed of 1250 fps,if you change shell speed to 1145 fps,how much is poi effected?
     
  2. hmb

    hmb Well-Known Member

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    Shell speed is only one element in a complex equation related to POI. The easiest way to find out is to shoot the gun at the patterning board. Many shooters are surprised when the slow shell has a higher POI than the fast load.

    Barrel length, target distance and type of sights on the shotgun all can have an effect on POI. HMB
     
  3. jimrich60

    jimrich60 Member

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    In theory, shot velocity can, of course, affect POI, much as bullet velocity affects rifle or pistol bullets. That is, the faster the shotgun shell, the "flatter" the trajectory would be (i.e., less drop at a given distance). However, given the small differences in shot velocity normally encountered, as well as the relatively short distances involved in clays shooting, make this pretty a moot point from a practical point of view. Even a change in POI of an inch (I don't think there would even be that much) would be of little consequence given the fact you are talking about a pattern of some 20-30 inches in the first place rather than a distinct singular projectile. While I have not done any real world patterning at a set distance with ammo of such different velocities (say 1150fps as compared to 1350 for instance), I personally doubt I would see much difference in POI at 40 yards as to be able to reliably measure it. Just my opinion.

    Jim R
     
  4. Neil Winston

    Neil Winston Well-Known Member

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    Not a measureable bit.

    Neil
     
  5. Hap MecTweaks

    Hap MecTweaks Well-Known Member

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    If you'd switch to a foster slug, 1 ounce or a 7/8ths, you'd see a small difference. The faster slug would drop a tad less in shotgunning distances.

    The low velocity shotshells shooting a higher POI? The ones I tested didn't. I measured to the centers of both velocities and found higher velocities spread a little more making it appear to shoot lower but the centers remained the same.

    Some get confused when the painted dot wasn't touched and the bottom of the low velocity pattern was above that dot. The fast load may have a few pellets in closer to that dot or some may hit it, then, some serious assuming takes place. Hap
     
  6. Too Bad

    Too Bad TS Member

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    According to Ed Lowery and Keith Garner program...ShotShell Ballistics

    The following are their results.

    Basic assumptions are......12 ga, 1 1/8 ounce - 7 1/2 shot - unbuffered, full choke, velocity 3' from barrel - the drop at 40' is:

    1150fps.............3.0"

    1250fps.............2.6"

    Tempurature and elevation effect drop, however the differance between the two speeds is still about .4". Higher tempurature, say 90 degrees at 5,000 above sea level such as Delta Colorado the drop will be.....

    1150fps.............2.9"

    1250fps.............2.5"

    Richard Luckett
     
  7. JACK

    JACK Well-Known Member Supporting Vendor

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    Prob a half pattern in your mind
     
  8. phirel

    phirel TS Member

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    As Richard pointed out, different air densities can change the total drop of both loads but the difference between the two loads should remain constant.

    I have for a long time had a question about confirming the extrapolated numbers generated by Ed Lowery and Keith Garner. I defy anyone to accurately measure the center of a pattern to an accuracy of 0.1 inch.

    Pat Ireland
     
  9. Too Bad

    Too Bad TS Member

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    Pat you are correct, but their work gives us something to think about. I suspect with today's technology, someone could accurately measure drop and drift more accurately.

    Shoot well............

    Richard Luckett
     
  10. zzt

    zzt Well-Known Member

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    The change in POI when switching between fast and slow shells, or between 7/8oz and 1 1/8ox loads is recoil related. Shot drop variations are too small to notice.

    For me, faster equals lower, by an amount noticeable on the line. Same with payload. The effects are cumulative. The difference in actual POI between a 24g, 1300fps shell and a 1 1/8oz, 1145fps shell are significant.

    The cause is the difference in recoil related upward rotation of the barrel during firing.
     
  11. phirel

    phirel TS Member

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    zzt- Your explanation does seem logical, but it may or may not be accurate. My question is when does the barrel begin to rotate upward. It may not begin until after the shot has left the barrel. This thought is based on what happens in my ported gun. The barrel always moves down, not up. It certainly is possible that there is a slight upward rotation before the gasses reach the ports and then the barrel moves down. Or it is possible, because of inertia, everything has left the barrel before the gun rotates in any direction.

    Pat Ireland
     
  12. zzt

    zzt Well-Known Member

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    Pat, my barrels are not ported. The recoil induced upward rotation of the gun begins before the ejecta leaves the barrel. If that were not so, I would see no difference in POI when changing speed and/or payload. I do see differences.
     
  13. hmb

    hmb Well-Known Member

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    zzt has it right. HMB
     
  14. bridgetoofar

    bridgetoofar TS Member

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    Open this link, scroll down and look at the drop chart. That should make it clear.

    http://www.shotgunsportsmagazine.com/downloads/shotgun_statistics.pdf
     
  15. trapster100

    trapster100 Member

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    If a rifle is shot horizontal (level) and a bullet was dropped at the same time, both would hit the ground at the same time. Gravity doesn't care how fast your going. The difference is the time to get there.
     
  16. Carol Lister

    Carol Lister TS Member

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    <blockquote>"If a rifle is shot horizontal (level) and a bullet was dropped at the same time, both would hit the ground at the same time. Gravity doesn't care how fast your going. The difference is the time to get there."</blockquote>

    As long as the height of the drop and gravity are constants the "time it takes to get there" will always be the same regardless of the velocity. The distance traveled by the fired bullet during that time will be the variable.

    Lister
     
  17. wayneo

    wayneo Active Member

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    If the shotgun fit to the shooter is correct, and the the gun placed in the pocket of the shoulder is correct, the movement of the gun should be in the opposite direction of the ejecta. Not up or down, but straight back against the shoulder at the same angle the barrel was pointed. Follow through is critical in wing shooting and clay sports. Do you see your barrel jump up or down on your follow through after you pull the trigger? I don't. Wayne
     
  18. Neil Winston

    Neil Winston Well-Known Member

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    There are two elements to recoil, according to John Brindle in his book. The first is a rotation about the center of gravity, the second also a rotation (up at the far end, like the first) as a result of the barrel-bore being above the shoulder when the gun begins to press there.

    When I did the test off a rest, using extra-lites and handicap shells there was no difference. This, I admit, is not a good way to do the test, since the gun is far better stabilized and rotation may be suppressed. I am not a good enough off-hand shooter to do the test that way, though.

    It's simple enough to do. All you need is some paper and shells; I recommend 20 yards, maybe 25 with a full choke. This is a good balance between giving enough distance to see differences (if there are any) and the ability to tell where the pattern-canter really is. You need to record _every_ shot and take a lot of them, say five or ten of each. Why doesn't someone do it and tell us how it came out?

    Neil
     
  19. phirel

    phirel TS Member

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    Some seem to be confusing the two independent factors in this question. One is the decrease in POI due to gravity. This can easily be calculated. The second is the possible raise in POI due to rotational forces. Neil correctly quoted John Brindle's description of two forces but there are other important factors. The angle between the gun butt and the shoulder and the angle that the shoulder is pushed back during recoil. I find myself leaning forward into the recoil when I shoot. When I get a bad shell, I stumble forward a step. The angle and the force of this forward leaning force would also have to be considered and now that takes us to body weight and foot position.

    I like numbers, but not enough to begin to try to calculate all of these vectors. Also, the first question that must be answered is when does the barrel begin to move. If barrel movement is suppressed by inertia until after the shot leaves the barrel, then nothing but gravity needs to be considered. The barrel movement seen in ported guns seems to suggest that this might be the case.

    Pat Ireland
     
  20. Dr A C Jones

    Dr A C Jones Member

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    "how much does shell speed efect poi? suppose you shoot a 100% high gun with a shell speed of 1250 fps,if you change shell speed to 1145 fps,how much is poi effected?"

    My guesstimate would be "not much" and "less than one need worry about". 1145fps vs 1250fps is about 8% change in recoil. Whatever effect the extra recoil gives, much more (~12.5 times more) is already present as a baseline.

    So, working in reverse, if the change in velocity caused a change in POI that people might be concerned about of 4" (say), then the baseline effect must be ~50". i.e. on a dead flat target going straight-away from the shooter they would have to "aim" ~50" below the target so that the gun rears-up into the correct position.

    If this was true, laser boresighters could never be useful.

    Further, once the shot leaves the bore, the gun continues to move (up as well as back). It is a guess how much movement occurs after the shot leaves the bore versus how much with the shot in the bore, but lets guess 2x. That means the gun finishes recoiling pointing ~100" above the target.

    Hence, in this example, the total movement of the POA is 150" (12.5-feet). This is much more than practical experience, therefore, the change in POI at the target must be very much less than the 4" assumed at the start.

    Andrew.
     
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