Effect of Bullet Spin on Terminal Performance

Discussion in 'Long Range Hunting & Shooting' started by Tiny Tim, Aug 17, 2019.


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  1. mwkelso

    mwkelso Member

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    This calculation is assuming a 1:12” twist...
    For quite a while I could not figure out where “720” came from. Turns out it’s simply 12*60, or 12 bullet rotations per 60 seconds for a twist velocity in rpm.
    Here is an article from accurateshooter.com that breaks down the theory behind twist velocity, and also goes a little deeper into the calcs behind it.
    https://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/
    To the OP:
    If your looking for optimal spin, you might consider looking at either Greenhill’s or Miller’s optimization equations. They have helped settle a lot of the controversy for me anyway. Not to disregard any of the above comments and suggestions. There is definitely a lot of wisdom in there, far beyond what I have in experience.
     
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  2. bubbaonpc

    bubbaonpc Active Member

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    Wow ! Now this has me wishing I'd gone further than 5th grade ! I'll be up half the night with slide rules...calculator and ballistic charts...nervously pacing my reloading room ! Darn you Tiny Tim !! Ha ! Great question...I think..............
     
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  3. doubledoc

    doubledoc Member

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    IMO, all factors would have to be considered as playing some role, rotational velocity, the shape of the expansion mushroom, the bullet materials and construction, and especially the velocity-hydrostatic shock factor. Obviously, a non expanded bullet would do little by rotating while an expanded bullet with exposed sharp pedals of Cu would have more effect. Intuitively, the major factor would be velocity and expansion, but in science, all these little factors play a part. There are far too many variables here to design a reliable simple experiment....
     
  4. GeorgeS

    GeorgeS Well-Known Member

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    Yes - the presumption, of course, is that the "black talon" holds together. Surprisingly, bullets hold together quite well. I have a load that uses 50 grain SBTHP bullets out of a .220 Swift, with a muzzle velocity of 4,400 f/s - got it out of an issue of Handloader back ages ago. In my 1:14 twist Ruger 77, that bullet is spinning 4,400*12/14 rps =226,286 rpm. Yet it holds together just fine to the target. At the target, however, it explodes violently from both impact speed and rotational force. In the old days, with less well-designed and manufactured bullets, .220 Swifts had a reputation for bullets blowing up midway to the target at only 4,000 f/s muzzle velocity.

    So the presumption in my post was that a well-designed modern bullet would hold together - particularly a skived monolithic - and wreak havoc throughout the wound channel. Your example of a 6.5 at 2,600 f/s out of a 1:7.5 would not only be spinning faster (268,000 rpm), but the larger diameter of the bullet would result in tremendous internal forces on the bullet trying to tear it apart - yet they hold together well. Modern bullets are very well built.

    It should also be noted that as a bullet expands, the expansion itself - owing to conservation of momentum - slows the bullet's rotation, not just the drag from the now-larger diameter. It's the opposite phenomenon we see when a spinning skater pulls in his or her arms, increasing the rotational rate of the spin, then extending arms to slow again.

    But there is no question that the energy imparted longitudinally by the bullet transit along its path within the target far exceeds the energy imparted by rotational effects. At in-target velocities above about 2,200 f/s, the hydrostatic effects are very significant.
     
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  5. mmgravy

    mmgravy Active Member

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    I have seen the same thing with my Swifts on prairie dogs. 1-8 performance is much more dramatic than the 1-14 with the same bullet....
     
  6. crkckr

    crkckr Well-Known Member

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    Ok, let's get this out the way first thing: a theory is a fact, subject to change with additional information. Someone's wild guess or even well thought out, but untested, idea is a hypothesis.

    Anyone that believes that rotational speed has no effect on an *expanding* or fluted bullet is just flat out wrong. While the Black Talon gel tests have been mentioned, does no one remember the chunks of gel being thrown out of the entry hole? I certainly do! As an aside, there is no significent hydrostatic shock from a bullet traveling less than (depending on who you believe) 2000 to 2300 fps. There is not enough pressure to actually damage flesh beyond bruising. Once the speed threshold is reached, flesh & organs will be torn and damaged by the hydrostatic shock.

    Watch the gel tests closely in virtually any of them and when the bullet expands there is always some additional damage done by the rotational speed of the bullet, which literally flings gel outwards from the center of the wound track (keep in mind gel and flesh do not act the same way). The fluted bullets, e.g. the Lehigh Defense XTreme bullets, depend on this rotation (and higher than normal velocity) to do the huge amount of damage they do to flesh. Shooting these 90 grain bullets out of my wife's Glock 26 (3.2" bbl) these +P+ rounds clock very close to 1400 fps! There is at least one vid on utube that shows a decent sized pig that was shot in the shoulder (after being shot in the neck and killed with a rifle) and the damage was nothing short of spectacular!

    All that said, I suspect that rotational velocity is more critical to bullet stabilization than it is to wounding a critter. Even if the bullet were not spinning it would still expand and cause nearly the same damage and if the velocity is high enough, cause hydrostatic shock along the wound track. While it may help, the difference would probably not be significant. But that's just my guess, I am not in the least bit confident that it is so!
    Cheers,
    crkckr
     
  7. Tiny Tim

    Tiny Tim Well-Known Member

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    MW, I believe the 720 in the formula is derived from 60 sec/minute x 12"/foot. The result is them divided by barrel twist rate. It simply gives a number that is a constant rather than constantly plugging in repetitive numbers. Thanks for the link. Good information there.
     
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  8. mwkelso

    mwkelso Member

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    Yes, you’re right. Looks like I let my typing get ahead of my brain. Thank you for correcting that.
     
  9. Lycanit

    Lycanit Well-Known Member

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    The effects are not only limited to rotational effect helping the expansion.. as peeling a banana.. it is also slowing the rotation as a skater would opening their arms. This effect also "throws" material or water..tissue.. outward also accelerating the hydrostatic effect and occasionally causing secondary projectiles.
    Remember the deceleration linearly is greater than the rotational slowing in tissue. This can be seen in clear gelatin as the rotation seems to compress in nature as travel slows quicker that rotation. Then again....lol...what do I know..
     
  10. Milepost

    Milepost Well-Known Member

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    I get that, and your exactly right! (spent 20 years in the military...)
    All very good points... You explained the mechanism for which how spin/rotation speed makes a modern bullets more terminal. This is a major component in the design of the bullet.

    It sounds like you are presuming that a modern day bullet was designed to hold together well after impact like a Barnes. Some do and some don't. There are many monolithic bullets that are designed to come apart using the centrifugal force of the spin rotation to create a larger wound channel in the boiler room such as the Cutting Edge Laser or Hammers. The Berger lead core bullets also exploit this concept to create massive internal trauma and it was designed right into the manufacturers design from the very start. However, these same extreme rotational forces work against a bullet when exiting a barrel at extreme high velocities and a lead core bullet can deform or comes apart. So for lead core bullets its all a compromise.

    The problem arises when we select a specific bullet/cal and then try to use it in a way that is outside of its intended design. Acknowledging these limits and selecting the right bullet for the job.... I've seen way too many hunters only work up one load/bullet and then try to use it for EVERYTHING! Of course the one load/bullet idea isn't really a bad idea because you become experienced and confident with that round and that converts over to accuracy. Accuracy, as we all know, trumps everything...but in the long range game the wind and rain will humble even the best hunter. So if your smart, you will select a a high BC bullet that is first priority accurate and also creates the widest internal wound channel. When your shot is off the mark all that flying shrapnel helps to make for a quick and humane kill. The fast spin rate is the mechanism for this type of bullet to do its work. But what do you do when the game your hunting is up close? That same ELR bullet may not be ideal for an under 300 yard point blank range shot. That's were a Barnes or similar bullet design works best... fast spin & fast speed=DRT!

    When I head out with my 300 Norma or 28 Nosler I may intend to shoot at extreme ranges but you never know!?! So now I always run with two loads. Two in the mag for surprise up close shots and two on the side for ELR.
     
  11. Wedgy

    Wedgy Well-Known Member

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    Rotation stabilizes the bullet so it can keep going straight once it hits the target. The 195 Berger got a bad reputation at first because people were shooting it with a 9 twist and although it was stable enough to make round holes in paper it would fold over, bend like a banana, and not open up when hitting game. The most stable way that bullet flies is backwards, not very aerodynamic but very stable. It is trying to flip over in flight but the spin prevents it until it becomes unstable/under rotated and you see it hit the target sideways on paper. It is impossible to keep going straight, expand(hollow point filling up) and create a wound channel if it's on its side. Think of a top spinning on a surface, when it's spinning fast you can't knock it over but once it slows down it gets wobbly and falls. The .30 caliber 230+ grain bullets can have this problem also when shot from a 10 twist. I have to shoot lead free in CA which requires a faster twist for like weight projectiles and have been addressing this issue for a while. Hopefully Steve from Hammer will chime in as he has some very interesting observations on this subject.
     
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  12. HARPERC

    HARPERC Well-Known Member

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    He's done a lot more penetration testing than most.
     
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  13. Bravo 4

    Bravo 4 Well-Known Member

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    I know this isn’t on topic with this thread, but wanted to know how successful you are at such ranges (especially first round hits). Theoretically, as you put it, we are talking several feet of drift or the equivalent to a couple mile per hour wind. Why not compensate for such a predictable displacement of the bullet? That’s what a ballistic app can/will do for you, every modern one I’ve seen has such a feature. Couldn’t you as the shooter be inducing error into the equation by simple neglect?
     
  14. Bravo 4

    Bravo 4 Well-Known Member

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    I believe I've read him post about faster twist equals potentially better terminal performance. Hopefully he will chime in.