Pressure flats

Discussion in 'ELR, Ballistics & Bullets Board' started by tomswede, Jan 8, 2018.

  1. tom

    tom Gold $$ Contributor

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    Rl16 had a flat then a big jump, and this nice flat up near max. The paper at 1,000 agreed so I was pretty happy, especially for a pencil barreled hunting gun.


    20170923_092131.jpg
     
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  2. Berger.Fan222

    Berger.Fan222

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    From a statistical standpoint, one would need to shoot a number of shots for each step in charge weight. This allows computing both a mean velocity and an uncertainty in each charge weight. Then when one looks at a graph of the plotted velocity vs. charge weight, the issue of a possible "flat" does not depend on several points being below the best fit line, but rather whether the computed uncertainties (error bars) overlap with the line.

    A more sophisticated (and rigorous) approach would be to fit the data (velocity vs. charge weight) to a third degree polynomial and determine if the uncertainty in the coefficient of the cubic term is significantly smaller than the cubic term itself. If it is, then the cubic term IS significantly different from zero, which would be solid evidence that an apparent flattening is not the result of a small sample size or other statistical anomaly.

    I've done a lot of work over the years addressing the question of whether given measured data sets have a constant first derivative: is the data linear, or is there clear evidence for a non-linear trend? The purported "flattening" in a velocity vs. powder charge curve requires not only a non-linear trend, it requires the first derivative to first decrease and then increase again. Compelling evidence for this will usually require at least 20 data points.
     
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  3. milanuk

    milanuk Team Savage Gold $$ Contributor

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    ...or a ladder test at distance ;)
     
  4. Berger.Fan222

    Berger.Fan222

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    I hope that is sarcastic.

    There are too many confounding factors at distance that render bullet drop an unreliable proxy for velocity. The shooter may not care whether the vertical tightening across a range of powder charges is due to velocity flattening or positive compensation, but a ladder test at distance cannot really be used to establish pressure or velocity flattening. You need a good chronograph and a statistically significant sample size for that.

    See: http://forum.accurateshooter.com/threads/positive-compensation.3890281/
     
  5. tom

    tom Gold $$ Contributor

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    Its

    I find the two usually correlate, it would require some effort for me to find a time they didn't actually . But that would depend on the definition of "flat spot". For me it means a dip in the linear progression of the velocities, at least for the purposes of this discussion.

    Tom
     
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  6. fredo

    fredo Silver $$ Contributor

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    I don't reference MV "flattening" so much as an indicator of an accuracy node, rather that it is another signal of that particular powder's velocity potential is close to peaking. In that regard, the only "flattening" of the curve that I take note of is that charge weight range is nearing the point of over-pressure & case damage/failure...
     
  7. retired

    retired

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    i do not know who berger222 is, i do know tom matt donovan and alex.
    i'll stick with their real life facts over someone else's math
     
  8. Berger.Fan222

    Berger.Fan222

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    The method some describe with a simple ladder test and a small number of shots can work well in the hands of experienced reloaders who combine careful component selection, brass prep, powder weighing, and bullet seating to reliably produce small extreme spreads in velocity for any given powder charge.

    But I've measured too many real F-Class and bench rest velocity spreads (LabRadar) to recommend that approach with much enthusiasm to a broader audience. Not everyone can effectively implement methods in the same way that the most expert and experienced reloaders can.
     
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  9. retired

    retired

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    and the volume of shooting recommended by you is impractical for the average shooter.
    the average shooter is not skilled enough for consistent loading and shooting.
    working with ladder data adds experience and improves the shooters skills.
    i like data, but the bottom line is real world shooting is what counts, ladder supports that.


     
  10. Berger.Fan222

    Berger.Fan222

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    Been practicing and teaching 20+ round load work-ups for over 20 years. My view is that beginning reloaders really need to load and shoot 5 rounds at each powder charge as they work up to look with enough care for pressure signs before incrementally increasing the powder charge. Shooting one round at a given charge is insufficient to gain confidence that there are no signs of excessive pressure before increasing the powder charge.

    I've introduced lots of new shooters to the sport. In addition to a bunch of average shooters, some of my more exceptional students have a combined 5 national championships, 5 second place finishes in national events, multiple cleans, two national records, and lots of victories in local and state events.

    Teaching a 20+ round load work up is not holding anyone back. We've found it to be very reliable for identifying the most promising loads in a variety of cartridges with a wide array of match, varmint, and hunting bullets. Slow is fast. Fast is accurate. Accurate is deadly.
     
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  11. CharlieNC

    CharlieNC Silver $$ Contributor

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    From my limited experience the target is more reliable than the chrono; in other words the SD of the shot (or group size if you prefer) vs the POI changes is less than the SD of the velocity as compared to the incremental changes in velocity due to charge weight. Not to mention the compensation due to harmonics can dominate the effect of the velocity deviations; isn't that really what we are trying to locate and harness as a node?
     
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  12. retired

    retired

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    so far all i have heard is you trying to justify your expensive process...a process you teach and i assume make money doing. so you have a vested interest in YOUR PROCESS. we ladder shooters are only interested in a personal progress, not filling our wallets.
    i have one national championship, lots of wins, over 40 years of learning how to do it "right".
    some how i managed without your 20 shot process.
    those who can, do
    those who can't, teach.
    you have a good day

     
  13. normmatzen

    normmatzen Silver $$ Contributor

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    I do believe it is not a myth. Our barrels have two distinct resonances. The first one is a linear resonance akin to an organ pipe. This is sometimes called the traveling donut. This resonance is a function of the speed of sound in the steel used for making the barrel and the exact length of the barrel. When the round is fired, a slight bulge goes down the barrel at the speed of sound in steel. When this bulge gets to the muzzle, it inverts phase as the barrel is open and becomes a constriction. When your bullet moves with an appropriate barrel time, when it gets to the muzzle about the time the traveling donut does, it slows down the bullet very marginally. at the load causing this effect, you will see a spread of loads where Mv is flat with load. So, at this load, the gun is less sensitive to powder lot and weight variations.
    But, there is another resonance which is the cantilever resonance where the barrel whips up and down (if your gunsmith clocked the barrel correctly) which when tuned correctly, alters the instantaneous angle of the barrel so there is a slight variation of Mv that puts the bullets in the same hole provided the change of angle sends slow bullets up and fast bullets down. That is positive tuning. If tuned for negative resonance, that Mv will cause splatter of your groups!
     
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  14. tomswede

    tomswede

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    Lots of interesting input. It seems to me that the “flat” is subtle but maybe it exists. It’s hard to imagine what causes it. Need to contemplate this a bit. For long range the SD needs to be low so chronograph data is needed for the chosen load. If shooting a ladder on a target, does trying to read the velocity increments add anything to what that target already says?
     
  15. tom

    tom Gold $$ Contributor

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    For me it is just more data. I owned an oheler for years, but never collected the data for target rifles, but did use it for hunting rifles. Now a days, the lab radar is painless enough to set up, I go ahead and gather the data. Tuning, and deciding on the load, for target rifles I go with what the paper tells me.

    Tom
     
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  16. milanuk

    milanuk Team Savage Gold $$ Contributor

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    Sorta my default mode... ;)

    In a more serious vein, it depends. You came across, or at least the way I read what you posted, as advocating 20+ shots. If you mean *per increment*... then yep, we're going to have to disagree. If you meant something else, please elaborate.

    Plenty of people have demonstrated that its very much possible to identify and tune to a 'flat spot' on target with a lot fewer shots per increment than that. I personally am uncomfortable with and/or have low confidence in doing a ladder with just one shot per increment, but that's based on my experience. Somewhere in there is a tipping point between 'enough data to be confident' vs. beginning to introduce un-necessary 'noise' from ambient conditions, shooter error, etc. And when I mean 'confident', I don't mean I'm submitting the data to a scholarly journal for peer review.

    Just in case it wasn't clear... I don't necessarily advocate 'run a ladder, and call it done'. I look at ladders as more of a sort of screening experiment, to narrow down the area of interest. I've seen various recommendations in texts on that sort of thing that suggest allocating about 25% of your 'budget' for this stage of your experiment/test. That sounds about right, by my experiences. Unfortunately there are times when I've 'spent' way more than that trying to 'make' a load work, when I probably should have just said 'enough' and moved on to something else more productive.
     
    Last edited: Jan 10, 2018
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  17. Berger.Fan222

    Berger.Fan222

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    No need for 20+ shots per increment. 3-5 shots per increment is enough to have a sense for what your velocity variations are at each point and whether the observed flattening is an anomaly or a real indicator of the response of velocity to increasing powder charge.
     
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  18. fredo

    fredo Silver $$ Contributor

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    I don't think anyone suggested that shooting 20+ individual ladder tests is the way to 'proof' a load. Rather that, a perceived 'flat spot' may not be as evident or pronounced when a larger sample size is examined...

    Reckon we can still agree that the target don't lie, and is prolly the most practical indicator of where a charge weight should settle at?
     
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  19. retired

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    i think you are mixing facts.
    ladder vs "Been practicing and teaching 20+ round load work-ups for over 20 years. My view is that beginning reloaders really need to load and shoot 5 rounds at each powder charge as they work up to look with enough care for pressure signs before incrementally increasing the powder charge"

    initial loads at 5 rounds is typically a waste of ammo. a 3 shot group never gets SMALLER WITH SHOTS 4 AND 5.
    latter he addmits to 3 shot groups, but that contradicts his "training plan".

    yes the target is the final answer.

     
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  20. damoncali

    damoncali Gold $$ Contributor

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    There is a difference between five individual 3-shot groups and five 3-shot groups that vary a single variable with a known behavior. The latter is what we're discussing here.

    I really don't understand the aversion that people have to shooting in order to gather data. Is it not practice? Does it not increase your experience? Nobody bashes practice or experience, but the second you recommend that statistical confidence is a good thing, the math haters come out of the wood work. Whatever works, I suppose. Different strokes for different folks.
     
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