WHY do velocity nodes exist?

Discussion in 'Reloading Forum (All Calibers)' started by chop house, Jul 9, 2018.

  1. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Flat spots on paper, small vertical, in spite of velocity changes. Agree on your last statement. PM sent
     
  2. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    OP,
    I dont know. But they do. Not with all powders and combos. To add to this, why does one powder charge produce good ES when the other doesnt?
     
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  3. Raythemanroe

    Raythemanroe Bullet Whisperer

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    A node is finding a tune (bullet,powder,primer,barrel etc) where it wants to shoot.. Usually this is the most forgiving spot in the tune that will tolerate a small charge fluctuations.. I like the ladder tune, easiest to see the overlapping In charge..

    Ray
     
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  4. damoncali

    damoncali Gold $$ Contributor

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    Was just about to say this, more or less.

    One is that as you increase charge weight, you will see a "flat spot" in a chart of charge weight vs velocity. I have never seen this in way that convinced me that it was anything other than random. If you shoot 10 rounds, you're very likely to see a flat spot even if the highs and lows are random. Others swear they're there. I'm not sure what you'd do with that information, but I've never messed around with it, because I don't see it in my rifles.

    Another is what al is calling "sine wave tuning". This is undoubtedly, take it to the bank, real. It's been documented thoroughly. Some call it positive compensation. If you shoot one shot, each one inch the the right of the last in increasing charge weights, you would see a stair step pattern - IF your rifle is accurate enough. Then, if you correct those points for variation in velocity drop, you'll find al's sine wave. The reason for this is that the weight distribution of the rifle is in the up/down direction - the scope is on top of the rifle, and the stock is below it. The recoil drives teh rifle in an up down pattern because of the inertia of the rifle above and below the bore. That's why the target pattern is up and down and not left and right.

    Then you get to generic load development. If you see the sine wave pattern described above, you might come to the conclusion that that is the only thing going on -that the barrel is whipping about in a sine wavey pattern up and down. But it's not. In reality, that is just one mode of vibration, albeit a dominant one. If you shoot enough groups, you might even find some that are horizontally bad, but vertically good. That's because there are other modes in there too - at different frequencies that only show up on the target under certain conditions. Load development as we know it is finding a combination that minimizes group size for all of the vibration modes at the same time. With very accurate rifles (PPC, etc), this is clearer than with less accurate rilfes, but you can see it if you know what to look for. I'll call this "tune".

    However, tune is not all there is to accuracy. It's very roughly only half the battle. Bullet quality is another major, if not dominant, factor. Thankfully most of the bullets we use are of very high quality - jacket wall thickness tolerances are obscenely small. They're very straight. Weights are consistent. But if any of those are off, your "tune" doesn't mean squat - the bullet WILL fly off in the wrong, unpredictable direction. We don't think about this much, because it's basically a solved problem - use good bullets. Twist rate matters as well. There's a reason that my .30 BR with an 18 inch twist is more accurate than my .308 wiht a 10 inch twist. Bullet length (I'm simplifying here) matters - this is another reason my .30BR is more accurate, and it's also why I can get away with the helpful 18 inch twist. For some reason people resist this idea even though it too has been thought through and carefully documented. Bryan Litz's book doesn't help, but it's not really his fault that people are misinterpreting his writing.

    And finally, I wish we'd drop the word "node" from our vocabulary. It's just not a useful term, and in no way does the node on a standing wave correlate to any of these concepts. It implies certain things that may or may not be true, and it's just confusing to people who don't know that it's shorthand for something else.
     
  5. ClassCat

    ClassCat Unlimited Gold $$ Contributor

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    upload_2018-7-10_11-28-11.png
    A node is a point along a standing wave where the wave has minimum amplitude.
     

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  6. barefooter56

    barefooter56

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    When I was with Sinclair International I took a Savage .223 and worked up powder charge velocity accuracy nodes going up 1 TENTH grain increments from the starting load to the maximum load just for curiosity. The targets followed the sine wave shape above. going from open to closing gradually until you hit a velocity accuracy node . Then starting to open again to the maximum group size . Then closing gradually again to an accuracy node. Took awhile to do but it was definitely a learning experience.
     
    Last edited: Jul 10, 2018
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  7. chop house

    chop house Silver $$ Contributor

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    thank you. when i posted i opted for the short form 'velocity node' as opposed to typing out something akin to "observed discontinuities in the charge weight vs muzzle velocity plot, which for the most part remains relatively linear in the area of interest ... that is to the point of overcharge and/or diminishing returns".

    anyway, although the replies took off on a few tangents, everyones input is appreciated.
     
  8. rkittine

    rkittine Gold $$ Contributor

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    When I was in engineering school in the 60s I had a great physics professor who loved demonstrations. Though not a rifle barrel, he took a long length of solid stainless still round stock and placed it TIGHTLY in a vise with one end extended somewhat like what we would see on a free floated rifle barrel. He than took a drilled out Cue ball that he had and put a string through it and supported it so it was hanging at the end of this barrel and just touching. He then took a schamy and started rubbing the barrel down almost the full length. Depending on the speed that he rubbed, the reaction by the suspended cue ball was different. The reason for the demonstration was to show what happens when something vibrates and hits resonance and when he finally got the stroking right the ball started shaking and finally shattered, just like a bridge with soldiers walking over in step.

    May not be appropriate to this conversation, but it did prove how much a supposedly rigid piece of steel could vibrate.

    Now being a newbie to this sport, I can only guess at most things based on my engineering background, but I would think that the velocity will for sure will result in different rotation speeds of projectiles and those projectiles may change stability by rotation speed.

    If you find a velocity node for say a 130 grain projectile and change to a different profile 130 grain projectile, would you expect the same velocity node, or that it MIGHT change.

    Bob
     
  9. TUFFLUCK

    TUFFLUCK

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    I've often wondered this as well. Short of an interior ballistics study by AB, Sierra, Alliant, etc....I think we'll be left with what the target tells us. But it's thought provoking to theorize. But I'd love to know what's really going on in that 2 milliseconds to cause this phenomenon.
     
  10. HillBilly98

    HillBilly98

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    Lay a ruler on a table and let it over hang part of the way and flip the end of it and watch it vibrate. While it is vibrating sling it more on the table and more off. The frequency will change.

    Try making wind chimes. The length is very critical for the proper frequency to resonate. At little to long or short will not ring but thud.

    That may not be the best examples but your barrel viberates ( some of the old writers called it whip). The rest of the gun viberatesas well. Some call it harmonics, some call it vibrations, it Symantec’s in my option with the terms.

    Not to make anyone who disagrees upset but it exist. Tuned loads can help to reduce them or even better time the exit of the projectile. SD is another important variable in that regard.

    There are other ways to deal with it within the rifle itself. One hack is the limb saver device. The BOSS system that Winchester and Browning come out with will also do the same.

    Sorry just my two cents
     
  11. mikecr

    mikecr

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    I'm sure there is a lot more going on than vertical whip.
    Even railguns have to be tuned.
     
  12. Romulus

    Romulus

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    For the target crowd I'm not sure Speed Nodes are the way to go as there are often much better groupings to be achieved through tried-and-true load development strategies, BUT for Tac-Match and PRS style competitions where travel and changing conditions is very common and utmost precision is not as important as understanding the game finding the most widest charge window often results in a very forgiving and quite accurate load across the competition season. No fuss and it works on the first or second range trip for me. I've been doing it this way for years when my time to tinker vanished and realized for most purposes a very forgiving charge weight that produces the same velocity variation across 0.5 to even a full 1 grain of powder is worth a lot in it's own right.

    When this strategy doesn't work for Tac/PRS guys I tend to think their barrel was not broken in before the testing or Margin of Errors in reloading precision or components are too high. That being said there is an entire group of reloaders who think because they trickle to 1 kernel of powder that they can run desired velocity and have a great load only due to consistency and the amount of $$$ they've spent on equipment. I run out of breath reminding these guys someone who knows what they're doing can usually tune a $900 rifle to shoot bugholes with a chargemaster and the cheapest dies and presses on the market.

    Most of the guys I know that do Speed-Node run a round-robin type of 2-3 shots per charge ladder looking for the wide flat spots when graphed. I was a stats tutor for a few yeras and know this isn't really that powerful of a sample but for many guts IT WORKS. I've used Excel for this for years and since no one wanted to hear about it, it was kind of my open secret.

    If I were to shoot F or BR I'd definitely look at other means of load development, but really that would be after I took a broad view of what charge windows were most forgiving, considering their utility in MV, and trying to get the windows to overlap with a great set of seating depth, charge, and neck tension providing good groups at the intended range(s) to get the best of both worlds.

    There are definitely spots in the charge window that shoot great on paper at short and medium but look like crap over the chrono and at 1000-1500 yds.
     
  13. Dusty Stevens

    Dusty Stevens COVFEFE Gold $$ Contributor

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    Time your bullet’s speed and the vibration it causes to make it exit at the same point in the barrel’s movement and you will have found the holy grail. And no matter how stiff the system is itll move like a wet noodle.

     
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  14. Someoldguy

    Someoldguy Silver $$ Contributor

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    I wish you'd not posted this. Now my head hurts. o_O

    The hell cartridge is that, anyway? Sure ain't no 5.56.
     
  15. alinwa

    alinwa

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    But there's a lot of video available....... I've got friends who film this hi-speed stuff. They ALL do that to a very visible extent.
     
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  16. Someoldguy

    Someoldguy Silver $$ Contributor

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    Understood, Sir!

    You're not the only one who yanks someone's chain, now and again. ;)

    We need another thread.

    'The He-Man Chain Yanker's Club'.

    I nominate Al as President. Anyone second?
     
  17. alinwa

    alinwa

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    huhh..... I done got took'n in there din't I

    Good On Ya ;)
     
  18. Someoldguy

    Someoldguy Silver $$ Contributor

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    I figured if anyone could take it, it would be you and/or Dusty.

    Just don't repay me in Spades, eh?
     
  19. alinwa

    alinwa

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    REPAY you?? Like vengeance and vendetta stuff??? Nahhhhh, vengeance is for pussies :)

    Besides, I don't yank nor pull nor even rattle them chains....... more a gentle TUG now and again.....
     
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  20. Someoldguy

    Someoldguy Silver $$ Contributor

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    Oy, Vey!
     

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