Bullet stabilization, strictly RPM?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by sambo3006, Oct 4, 2007.

  1. sambo3006

    sambo3006 Well-Known Member

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    Let's say that you have two different cartridges like the 6.5-284 and the 6.5 STW. Also assume that the same bullet is being used like the Berger VLD 140. Lastly, assume that the rifling twist rate on the two barrels is such that they both spin the bullet at the same RPM--faster twist on the 6.5-284.

    Is the bullet equally stabilized by each cartridge due to the same RPM spin even though it is most likely traveling at 300 or more fps faster out of the 6.5 STW? Will either bullet lose its rate of spin faster than the other in flight? I realize that the bullet from the 6.5-284 will slow to transsonic first, I'm talking strictly supersonic.
     
  2. goodgrouper

    goodgrouper Well-Known Member

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    No two barrels can be counted on to equally stabilize bullets by simply matching their RPM's alone. Even with proper twists for the bullet at hand, one barrel will treat the bullet different than the other even if the RPM's are the same.

    Stabilization is more than just cranking up RPM's to a certain number. It matters more that the yaw is reduced as much as possible as fast as possible and that comes from twist rate or distance covered by one revolution and also a host of other things. RPM's of course are intertwined in this equation but are not the direct comparison.

    In other words, you could get a Berger 140 6.5 mm bullet to stabilize in a 10 twist by pushing up the speed to attain a specific RPM and even fire it in a thin atmosphere and it would work, but it would be better served to just use the recommended twist of 9and keep the bullet in it's factory design specifications. I have always seen this to give more accurate groups.
     
  3. Mikecr

    Mikecr Well-Known Member

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    This is true.
    Stabilization follows displacement per turn, NOT turns per time.
    In your example above I can demonstrate why anyone should forget RPMs -forever.

    140vld at 3000fps from a 8.5:1 barrel would have an Sg~1.40 under ICAO conditions. This happens to be 254Krpms.

    Same bullet at 3300fps would be spinning 254Krpms from a 9.4:1 barrel. But under same conditions this twist rate would produce an Sg of only ~1.15

    At 3600fps, 254Krpm would come from a 10.2:1 barrel. this is the point where the bullet performance completely sucks though, as Sg would be ~1.00 at ICAO.


    So stability here is greatly affected regardless of rpms..
    It is DISPLACEMENT PER TURN that has by far the largest influence on bullet stability.
    Also, another factor not accounted for from an rpm perspective, is air density. This can wildly affect stability as well. It is the second largest influence, followed by muzzle release quality, and then dynamic design issues.

    Any connection with RPMs is pure coincidental.
     
    Last edited: Oct 5, 2007
  4. overbore

    overbore Well-Known Member

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    Sg

    I happen to use a Sg of 1.35 to consider a bullet fully stabilized for target purposes..Military is using much lower value for their 6.8 trials expecting that the bullet may tumble upon impact thereby going back to the people popper abilities we had early in Nam until the REMf's decided to classify helmets as enemies---. Overbore
     
  5. sambo3006

    sambo3006 Well-Known Member

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    Makes good sense. I hadn't thought about it enough to consider distance traveled per revolution based on velocity. A clean bullet release is also obviously very important. A buddy of mine found this out when he took it upon himself to shorten the barrel on his Ruger Super Redhawk without recrowning it and then couldn't figure out why his groups went to crap.

    I have a 700 7mm RUM that shoots nice small groups around 0.5" or less at 100 yds with 150 gr BT's. I believe the factory twist is 1-10". I am thinking about putting a brake on it and making it my long range deer/prairie goat/elk rig. Sounds like it probably would not stabilize some of the heavier pills in the 180+ gr range. I would think the 160 gr AB would stabilize nicely, maybe the 168 gr VLD.

    As an aside, does a longer more aerodynamic bullet require a faster rifling twist on average? Comparing say that same 6.5 mm 140 gr VLD with its listed BC of 0.64 and the 140 gr Sierra spitzer boat tail with a listed BC of around 0.49. Does it go back to the issue of the longer bullet being more prone to yaw?

    Thanks for the excellent feedback, guys!

    Sam
     
  6. dwm

    dwm Well-Known Member

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    As an aside, I think a lot of people get confused because a top spinning on the ground will stabalize with no forward velocity, IE based solely on rpm, and think that a bullet in flight will also ...
     
  7. goodgrouper

    goodgrouper Well-Known Member

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    The longer the bullet, the faster it must turn in displacement to stabilize it. Typically, the longer a bullet is, the more aerodynamic it will be if shapes are same but if you change the shape to say a flat base round nose bullet the aerodynamics will obviously suffer.
     
  8. Michael Eichele

    Michael Eichele Well-Known Member

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    Also to be conciderd is bullet construction and materials used. Bullets of the same length yet one being jacketed lead for example and the other being solid copper will not stabilize at the same twist or RPM despite being fired at the same velocity. Solid copper for instance has a much lower "specifec gravity" function than other materials and needs more twist to properly stabilize them.
     
  9. dwm

    dwm Well-Known Member

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    I had one more thought for this, if you have two of the same bullets going the same forward velocity, but one with a higher rpm, does one have a higher terminal energy?
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    Typically yes due to the extra cytrifical force that creates more of a shock wave therefore more temporary cavitation during the bullet's penetration. Over spinning a bullet brings other issues up though such as a reduced BC and the faster you over spin them, the more BC you will loose. Excessivly low RPMs on the flip side can give you a BC loss as well albiet the BC loss is caused by factors different than that of an overspun bullet though. For overall "in flight" ballistic performance you dont want your stability too great or riding the edge either. But yes, faster RPMs tend to cause more trauma in game.
     
  11. blackco

    blackco Well-Known Member

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    WOW!! Lots of information and just one mure subeject in this shooting arena that went right over my head right out of the gate. I do enjoy the learning from those who know.

    Thanks
     
  12. X-man

    X-man Well-Known Member

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    Blacko
    I too have learned a great deal here.
    Thanks to those that make this a great site!

    DWM
    You got me thinking about this rotational killing power stuff!
    I don't know how to calculate the energy of a rotating object, but here is what I tried based on the outside velocity of Mikecr's 140 grain .284 bullet at 254K rpms.

    Diameter (.284) X 3.143 (pye)= 0.893 inches (traveled per revolution)
    0.893 X 4233 revs/second (254K/60) = 3780 inches traveled/second = 315 feet traveled per second.

    While a 140 grain bullet traveling at 315 fps produces 30.9 fpe (not much) I can't see how a rotating mass could generate any more than this.

    As near as my pea brain can tell my calculations give credit to the entire 140 grains at the outside edge of the projectile (the center portion of the bullet is rotating slower than the outside edge). Which leads me to believe (in my pea brain) that the actual energy figure is less than I have calculated???

    There must be someone smarter than me that can make some sense of this, but according to my figures rotational bullet energy should have almost no effect on killing power???


    Now my head hurts......Help!!???
     
  13. Jon A

    Jon A Well-Known Member

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    The kinetic energy due to the spin of the bullet is negligible. However, if it helps initate expansion (a bit too much spin and many bullets will explode in mid air) I could see it providing a noticable improvement in some cases--especially with match-type bullets.

    For monometal bullet fans, such as Barnes, GSC, etc, I have found the heavier, longer (higher BC) ones can have a tendency to turn and veer off course while penetrating. Extra spin can help here too.

    For the above reasons, I'd tend to lean toward a step faster twist for a hunting rifle than I would for a purely paper punching rifle.
     
  14. blackco

    blackco Well-Known Member

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    Ok. Maybe I'm being obtuse but to help me understand some of what is being said; what is Sg and ICAO? Also, can someone dumb-down what is meant by dispalcement per turn?

    THis is a great thread but I'm getting an ice cream headache trying to wrap my brain around it.
     
    Last edited: Oct 26, 2007