Bullet rpms

Discussion in 'General Firearms Forum' started by Atomic Punk, Dec 7, 2019.

  1. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    I think he writes a new "set of chapters" once a year. They aren't really "editions" as each one covers new material/data.
     
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  2. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    Unfortunately, in practice there are so many variables - some of which aren't [1] well understood, [2] practically measurable and/or [3] predictable - that a lot of the information must be empirically derived.

    On the other hand, I guess that is pretty fortunate for Brian Litz, because he gets to make a living on something he has a passion for.
     
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  3. BigMoneyGrip

    BigMoneyGrip

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    I used to race dragbikes. I always told my buddy that RPM's were good. Then one time, on a nice summer day, two rods let go at about 14k RPM's. The rods that decided to call it quits weedeatered to block like a band saw on meth. It was then that I learned that RPM's could be your friend or your worst enemy. That day, I put the chip back in the MSD box and shifted before the the shift light illuminated.
    Not that this has anything to do with bullets, but it does have something to do with RPM's.
     
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  4. Doc Holliday

    Doc Holliday CLM

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    Wow. Did not know. Always assumed the shorter barrels couldn't do it. See, you really DO learn something new every day. Thanks.
     
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  5. AK_Stick

    AK_Stick AAAMAD

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    You’ll start to fall off the stability wagon about the time the bullet goes below supersonic.
     
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  6. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    Well, not really. As explained an earlier post, the transonic region introduces gyroscopic precession/yaw because of center of pressure shift forward (of the projectile), below the transonic region (below about 0.8 Mach) the yaw is dampened and stability is regained.

    Even though the dispersion is increased, the bullet will still be (gyroscopically) stable below subsonic velocities. Also, as explained earlier, the amount of dispersion effects high SD Bullets more than low SD Bullets; considering that low SD bullets are not commonly used at longer ranges, so it is kind of a moot point in long range/high power/precision rifle.

    If you want to get into a p1$$1ng contest about it, get into it with B. Litz, as this information comes (pretty much) directly from his books and seminars.
     
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  7. AK_Stick

    AK_Stick AAAMAD

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    You’re correct, I was overly simplistic in responding to the OP’s apparent link of stabilization to accuracy.
     
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  8. Dave514

    Dave514

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  9. JohnnyE

    JohnnyE

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    Here's a bullet disassembling itself with too much RPM.
     
  10. jbmillard

    jbmillard

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    My modified point mass calculator (http://jbmballistics.com/cgi-bin/jbmmpm-5.1.cgi) models it with pretty good accuracy. If you run it with the default inputs (0.308 168 gr), 2600 ft/sec, 12" twist, you'll find that RPMS start at 156000 and go to about 130000 at 1000 yards. In the same space, velocity decreased from 2600 to about 1030 ft/sec.

    This is why for most bullets, if they are stable at the muzzle, they are stable down range. Of course weird things happen in the transonic region so all bets are off there.
     
  11. FullClip

    FullClip Native Mainiac CLM

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    :goodpost:
    Thanks for the link...pretty amazing calculator...more data to fill in than most IRS Tax forms. Looks like some pretty serious stuff put together by someone way smarter than me.
     
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  12. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    Pretty good article, I hadn’t read this before. Some folks believe that, in practical application, a bullet can’t be “over stabilised”. I guess we would have to ask Al Gore if this is “settled science”.
     
  13. Dave514

    Dave514

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    I don't think Lateral Throwoff is that hotly debated.
     
  14. AK_Stick

    AK_Stick AAAMAD

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    I would counter that it’s possible, atleast in some cases, such as M2’s, were the bullets were spun so hard as to fail in flight, or in contact with a target.


    Though it would be interesting to shoot them into gel and see how they performed.
     
  15. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    There is a difference in “over stabilized”, some thinking Sg> 2 (with some thinking > 1.6), and bullet failure from centrifugal forces causing > than yield stress in the jacket; think of it as “over spun” for the bullet construction rather than over stabilized.

    I have heard opinions about the early M16’s with 1:14 rates were more effective in wounding because the bullet was less stable, and upon entering the body destabilized quickly and produced larger wound tracks. Than again I’ve heard that this theory “doesn’t hold water”.
     
  16. AK_Stick

    AK_Stick AAAMAD

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    I would argue that if we’re talking in practical terms, a debate between over spun, and over stabilized is largely academic if your bullet is failing upon reaching the target due to your rifling.

    As for the second part, I’ve heard that too. Which was why I said it would be interesting to see bullets such as What M2 describes shot into gel.
     
  17. PEC-Memphis

    PEC-Memphis Scottish Member

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    Doh ?
    The context was that “overstabilization” creates greater dispersion, not jacket failure. So it is not academic, two completely different subjects.

    Jacketed rifle bullets aren’t coming apart at 1.6-2.0 Sg.
     
  18. AK_Stick

    AK_Stick AAAMAD

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    Ok. Never mind
     
    Last edited: Dec 8, 2019
  19. NoStress

    NoStress

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    A bullet will still be spinning when all of its forward velocity stops and it falls to the earth. Bullets over spin when the velocity falls off so a bullet losing its velocity and overspinning is the problem. A bullet that starts out rotating one times in 10 inches will soon be rotating one time in less than 10 inches and this difference will increase with the range and amounts to an over spin at long range. Weather the muzzle velocity of a bullet is 1000 or 3000 fps, a bullet fired from a barrel with a 1 in 10 pitch will make two revolutions in 20 inches. But the 3000 fps bullet makes those two turns 3 times faster because it travels the twenty inches in 1/3 the time. In general, A heavier bullet holds its velocity and does not suffer as much from over spin as a lighter one.
     
  20. Rick James45

    Rick James45

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    I have some good books and DVDs from Bryan Litz's . He really is a Rocket Scientist. No Joke.
     
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