Change of POI

Discussion in 'Practical Precision--PRS, NRL, ELR' started by warbird2006, Apr 25, 2017.

  1. warbird2006

    warbird2006 Silver $$ Contributor

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    A point of impact with different ammo.

    I shoot 308 Win and 168gr A-Max with 42 gr of IMR 4046 up to 500 yards. I had to switch the bullets for a better knockout power of silhouettes on 500 yards, so I developed a load using 178 SMK with 43.3 gr of Varget. Both loads are similar velocity, BUT, the SMK loads shoot about 2-3" to the right then Amax on 100 yards. So I zero the scope, fire 20-30 shots (with pauses of course) and switch back to Amax, and they are impacting 2-3" to the left, again, 1/2" groups?
    Does anyone have an explanation for this change of impact sideways (and no, it's not the wind)?
     
  2. Nowhere Man

    Nowhere Man

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    I experience similar results with 185 juggernauts hitting the target 1" to the left at 100 yards. I believe it's barrel harmonics. I dunno.
     
  3. jds holler

    jds holler Gold $$ Contributor

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    It's very common, and I hate it.:( jd
     
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  4. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    It's very common...in fact, it would probably be the exception to the rule if you didn't see it. Barrel harmonic movement isn't solely in a perfect vertical plane, there is a horizontal component to its harmonic movement. In addition, two bullets of different weight that have the same muzzle velocity by definition have different acceleration in the bore; i.e. different barrel occupancy times. That means that even though they leave the bore with similar MV, the bore will not be at the point in the harmonic cycle when the two bullets exit. Therefore, depending on when the bullet exits the bore, where the bore is actually pointed will change. You have simply identified the scope correction that accounts for the difference between the timing that the two bullets exit the bore, and the corresponding launch angles.
     
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  5. Eternal Student

    Eternal Student Silver $$ Contributor

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    Excellent point. I read a post or book on something called the "shockwave theory" and it explained MANY things (such as barrel whip, time in chamber, etc...) that could affect POI. The smallest amount of time could create or destroy an accuracy-node. JMO I found it an interesting read but I don't know much about anything and I am easily fascinated.
     
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  6. MagnumManiac

    MagnumManiac

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    The reason is simple, we zero our rifle to a certain POI, this happens to be when the barrel is pointed in that direction. A lot of people also encounter a POI difference to where the scope was pointing when the bore was aligned with the scope, this is normal also.
    When we change a load, or a bullet, the barrel will most likely be pointing in a different direction to the previous load or bullets exit point in the barrel whip.
    A barrel moves in an oscillating circle during firing, how we fine tune a load is to find the sweet spot that ALL bullets fired exit the barrel at the same point.
    Barrel time and seating depth play the biggest part in altering the exit point of a bullet in this movement.
     
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  7. lloydx2

    lloydx2

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    I am sure someone will chime in and disagree with me on this......
    I always time barrel curvature at 12-6 With the idea of keeping harmonics more vertically consistent
    Lloyd.
     
  8. T-shooter

    T-shooter

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    I just ran into that. I used usually 3 different bullets. 208g A-Max/ELD now, 168g Hornady Match and Sierra 168g MatchKings. All of these shoot horizontally almost exactly the same. The 208g needs 1/10 mil left at 215 yards. I just started working up a load with Berger 200.20X 200 grain. Using the same powder charge and seating depth off the lands, they shot over 4" to the right. Even working the load up by nearly a full grain of powder, it's still hitting nearly as far to the right. No big deal, just harmonics and easily adjusted for as long as I keep records.
     
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  9. Bart B.

    Bart B.

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    I've not seen any change in windage zero shooting 150 through 200 grain bullets from 308 Win cases in 24 to 28 inch barrels with all sorts of weights and profiles at ranges through 1000 yards. This is shooting in field positions.

    Shot with the shouldered rifle resting on bags atop a bench, windage zero's are about 1 MOA left.

    Barrels have resonant frequencies of several dozen cycles per second. And at least eight higher harmonic frequencies up to a few thousand cycles per second. It varies with length and profile.

    http://www.varmintal.com/amode.htm

    I'm convinced the most common cause of a given rifle-ammo system having different zero's is how the rifle is held. Same rifle shot by several people will have different sight settings for range zero with the same ammo. I've seen a 2 MOA spread across four people. It's not because each one "looks through the sight differently;" that's a decades old myth.
     
    Last edited: Jul 11, 2017
  10. T-shooter

    T-shooter

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    I haven't seen it before until now. I can normally shoot 1/2 MOA at that distance. I fired 2 fouling shots to warm the barrel, fired 4 of the A-Max rounds which hit in .6 MOA centered on the target, then 5 groups with the Bergers. All 5 were off towards the right by 3-1/2 to 4-1/2 inches. The major difference in these bullets is the shape of the ogive. I use 41.5 grains of Varget with the A-Max. I went from 41.6 to 42.4 in 2/10's increments with the Bergers (so far until I get another chance to test)
    Compare2.jpg
     
  11. Bart B.

    Bart B.

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    I've no idea why the Bergers went right. I don't think it's due to bullets. Body torque during barrel times of both is virtually equal with both. Center of mass from recoil (bore) axis can cause horizontal group shift if different for each group

    I wonder where another group of the AMAX ones would go.
     
    Last edited: Jul 11, 2017
  12. T-shooter

    T-shooter

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    I've shot them before and they were ok. They are now replaced with 208g ELD's but I haven't fired any yet. The Amax along with 168g Hornady match and 168g Sierra M/K's all shoot the same horizontally within 1/10 mil. Just one of those things I guess. The Bergers have 1/2 the bearing surface, a longer ogive profile, and don't extend completely to the rear of the neck. As soon as the rains stop and the mud dries in central Ohio, I'll get these dialed in.
     
  13. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    The barrel doesn't vibrate in a perfectly vertical plane. Where the muzzle is pointing when the bullet leaves the bore affects its POI. Unless two bullets have the exact same barrel time down to a very small fraction of a millisecond, the bore will not be pointing in the same direction when they emerge. Both vertical and horizontal changes in POI between two different loads can be attributed, in part, to this phenomena. The two different bullets don't have the same barrel time, and the muzzle is pointing in a different direction when they exit the bore.
     
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  14. DHuffman

    DHuffman Gold $$ Contributor

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    A common practice but it has little to do with the OP
     
  15. CharlieNC

    CharlieNC Gold $$ Contributor

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    Doing a ladder test to find the node based on vertical poi, I don't recall a time that the horizontal did not move and converge at the same node. Demonstration of the same harmonic action. Its reproducible, and can be added to your dope when switching ammo.
     
  16. Bart B.

    Bart B.

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    I assume you mean when the muzzle axis is at the top or bottom of its vertical whip cycle. And if you "time" it, are you measuring the bullet's time from case mouth to muzzle exit? How are you measuring the muzzle vertical whip frequency and what point in its cycle it starts at and what frequency is it when bullets exit at either extreme?

    My thoughts on this 12 and 6 o'clock points of the muzzle axis cycle over time are the worst places for bullets to exit. As there's a spread in both bullet barrel time and muzzle velocity as well as pressure curve shape (that causes slightly different barrel time for the same muzzle velocity), only about half the bullets will leave at an angle to compensate for their velocity. Slower ones at higher angle, faster ones at lower angles; relative to the line of sight.

    This is called "positive compensation" and is why a sliding weight on barrels near their muzzles are used to "tune" the muzzle vertical whip frequency to compensate for velocity spread. All bullets leave on the muzzle axis up swing at the point where angular rate-of-change matches velocity change vs bullet drop at target range. This is what Browning's BOSS does.

    Note the barrel's several vibrating frequencies (several dozen to several thousand cycles per second) are exactly the same for every shot fired. Barrel metallurgy and shape are the same every time. Only their amplitudes change across reduced to maximum loads. Shown in this link:

    http://www.varmintal.com/amode.htm
     
    Last edited: Jul 12, 2017
  17. dmoran

    dmoran Donovan Moran Silver $$ Contributor

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    @Bart B.
    Think your taking @lloydx2 input out of context and meaning. Timing (or indexing) barrel curvature is done when fitting a barrel and is fairly popular practice with competition gunsmiths. There is a bunch of threads in the Gunsmithing Forums on the topic.


    @lloydx2 -
    Besides for the obvious windage centering and elevation gains for the scopes, I also time my barrels for the advantages you suggest. My own barrel spin testing has proven that indexing a barrel's curvature is effective.
    Donovan
     
    Last edited: Jul 12, 2017
  18. Bart B.

    Bart B.

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    Thanks for the clarification. All the 'smiths I chatted with on that issue called it "clocking in" the barrel so its witness mark aligned with that on the receiver. Unless you're referring to a bent barrel's high or low point.

    And the "6/12" issue often refers to bullets leaving at the bottom and top of the muzzle axis whip cycle.

    There are no word names, terms nor conventions consistencies in this forum regarding arms, ammo and their accessories..
     
    Last edited: Jul 12, 2017
  19. SheepDog

    SheepDog Silver $$ Contributor

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    There are a lot of variables between two different bullets and the way they can affect harmonics. It could be that minor powder charge adjustment could get the bullet to hit closer to the same point of impact as the lighter one. Minor powder charge changes could also throw the bullet farther away in a different direction. If you are happy with the group size then just make a note of the different sight adjustment and chalk it up to experience.
    Even though the muzzle velocity is close to the same the rate of acceleration is likely very different and that changes the harmonics.
     
  20. Bart B.

    Bart B.

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    The harmonics (frequencies) of the barrel are fixed; they don't change; they're determined by barrel mass and shape. Ammo velocity variables change where bullets exit on the muzzle angle range. If the average exit point moves in the middle section of the muzzle angle upswing, it often does not change accuracy but groups will typically center at a different place vertically.

    Which is why the same lot of ammo may have a 50 fps or more spread in average velocity across several barrels all with the same profile and weight. Yet shoot with equal accuracy in all.
     

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