Rotational velocity vs wound severity

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by elkaholic, Oct 26, 2012.

  1. elkaholic

    elkaholic Well-Known Member

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    I've decided to start this thread because I feel there is need to know "why something happens", not just that it occurs. As most of you already know, I make my own bullets and do a lot of testing, which makes this of particular interest to me, but applies to everyone who hunts. Recently, I tested some 215 Berger Hybrids for the purpose of hunting application. My goal was to ascertain what minimum expansion velocity was with this bullet to apply to long range hunting. I did this using reduced velocity loads fired point blank into water, which I have found to be a good testing media. The results were not what I had hoped for with more velocity needed than what I consider good long range performance. Brozs' testing on real life targets does not seem to duplicate what I found in my testing, and I am confident that water testing is not the problem! I have come to the conclusion that the rotational velocity difference between the reduced load, and a full throtle load, may well be the difference. How much is due to the rotation itself, or the additional expansion caused by the rotation, is what I would like to discover. I have done a little research and have found there are others who feel that rotation is definitely a factor. Adam gubar, whom I think wrote for Chuck Hawks, addressed this. If I can find it, there was an article written by the military years ago comparing damage between the M1 carbine and M1 rifle, both firing 110 grain bullets, the former point blank with a 16" twist and the later firing at a range to duplicate the impact velocity but with a full throtle load with a 10 twist. The resulting damage with the 10 twist was considerably greater! This was done in the ballistic gel which was used at the time. I'm not sure when I will get the time, but my plan is to perform the same type of test, only use the same rifle, one with a reduced load, and one with a full load but at long range. This should eliminate some of the variation which should be expected from different rifles. The rotational velocity does not decrease nearly as fast as forward momentum, so this should work. Also, I want to test both full jacketed vs expanding bullets, which should give a good indication of how much of the difference is due to expansion, and how much is due to the rotational energy itself. Besides the evidence of what I have read over the years, my personal experience tells me that there is evidnce of "more twist, more damage"! Some of this no doubt is due to twist on the integrity of the bullet jacket, but I think there is more to it than that. I welcome everyone to research this some more, which I intend to do, and post what they find........thanks/Rich
     
  2. phorwath

    phorwath Well-Known Member

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    I expect your thought process will be proven correct. Higher rotational velocity will result in greater bullet expansion, given equal impact velocities. And even if bullet expansion is not increased, I still believe the wound channel and trauma caused by an expanded bullet rotating at the higher RPM will result in greater tissue damage as the expanded edges of the bullet rotors through flesh and organs.
     

  3. royinidaho

    royinidaho Writers Guild

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    Watch some videos of bullets entering gel. They seem to make only a couple of revolutions.

    I got chewed out pretty well by posting results of expansion tests using reduced loads to get LR velocity @ 200 yds.

    People jumped all over the reduced spin rate.

    Personally I don't think it makes a squat of difference.
     
  4. Buffalobob

    Buffalobob Writers Guild

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    Jacket integrity and structural integrity are a big factors for the heavy for caliber bullets with thin jackets that are spun at high rpms.

    FMJs that are are at marginal stability (rpm) produce larger wounds than highly stable ones. This has been well documented by the military trauma doctors.

    The question you inquire about has many different answers that depend upon the initial construction of the bullet.

    There is an old thread by Buffalorancher that has some intelligent remarks in it and one excessively stupid one near the end. Watching Youtube cartoons when you can't fathom high school geometry has its problems.
     
  5. elkaholic

    elkaholic Well-Known Member

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    I suspect the marginal spin produces more trauma because the bullets are more apt to tumble. Unexpanded bullets also seem to tumble more. I I have found SMK's are worse than VLDs for not expanding, which also causes worse tumbling and banana bullets. This happens nearly everytime when a SMK doesn't expand. I believe this is caused by the greater angle ogive not allowing expansion to start as easily. Also, it would not surprise me at all if the Bergers which are causing larger exit holes at low velocity are the result of tumbling rather than expansion. Obviously, there is no proof on my part because the bullets are not recovered........Rich
     
  6. Browninglover1

    Browninglover1 Well-Known Member

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    Rich,
    So I can verify I understand your test method... you want to test the same bullet at the same impact velocity but with different RPM's to compare what happens. To do this you'll fire a reduced load at close range to achieve an impact with a velocity of "x" with a low RPM compared to a standard load that you'll shoot at long range to achieve the same impact velocity but higher RPM's. Is my understanding correct?

    I'm excited to see what your results are.
     
  7. elkaholic

    elkaholic Well-Known Member

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    That is exactly right!
     
  8. Buffalobob

    Buffalobob Writers Guild

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    Bullet deformation on impact is important for FMJs!!!!! It screws up the rotation and the bullet loses spin stability.

    On an expanding bullet, the center of mass moves forward as the bullet expands creating or adding stability to the bullet. Even in the absence of spin an expanded bullet will be stable passing through a body. Simple physics of the center of mass and stability. It is what archers call Forward Of Center and why an arrow can be stable without spin.
     
  9. elkaholic

    elkaholic Well-Known Member

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    Exactly.....Rich
     
  10. elkaholic

    elkaholic Well-Known Member

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    I just located the article written by Adam S. Gubar entitled (terminal ballistics- getting past the bs) It was under (hunting bullet terminal ballistics)
     
  11. Lefty7mmstw

    Lefty7mmstw Well-Known Member

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    I was under the impression that rotational velocity slowed in tandem with linear velocity. a bullet turns say 1 turn in ten inches at muzzle, shouldn't it be 1 turn in 10" at some far off range also, or even less since air resistance would act on the land cuts on the bullet surface. And that 1 in 10 inch linear is going to be slower at distance which would slow your rpm down. The two different progressions are locked together in the same ratio via the fixed bore twist. edit- at first anyway.

    An easy play on this would be to use two different 22 cal centerfires since different twist rates are so common.
     
    Last edited: Oct 26, 2012
  12. elkaholic

    elkaholic Well-Known Member

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    The rotational velocity and forward momentum change at a far different rate with rotation winning out. i.e. the forward impact velocity could be down 50% but the rotational velocity is probably still over 90%. There are charts which show this and I don't know exactly what the ratio is, but it is a lot!.......Rich
     
  13. Lefty7mmstw

    Lefty7mmstw Well-Known Member

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    interesting...
    air resistance in play on the forward axis but not as great on rotational.... feels a bit warped doesn't it.
     
  14. Mikecr

    Mikecr Well-Known Member

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    Normally stability climbs downrange, because displacement per turn(to overcome it) greatly decreases with falling velocity.
    So for example, a 10" per 1 turn at the muzzle ends up way less displacement per turn at 500yds.
    It ends up ~7.3:1 at 500yds with a 95gr BIB launched at 3kfps in 10tw. And Sg from 1.5 at the muzzle would increase to 2.85 by 500yds.
    This reverses trend once nearing mach1.

    I know terminal ballistics represents a very complicated science of it's own, and I'm only a varmint hunter. But what I have noticed is that speed of death relates to how much energy is released INSIDE game. This, moreso than the wound itself.
    I kill groundhogs far more directly with light bullets that leave no exit wound, than with heavy bullets which make a holy mess of the hogs(like several airborne pieces). I think this is due to hydraulic shock that is REDUCED when a bullet destroys any containment of it. And with this condition the game is allowed to bleed to death. With lighter bullets, releasing full energy that remains inside, the game drops without a twitch(not even a death kick).

    I saw this same result(in a video) with an elephant shot in the ass. It dropped instantly without a twitch, seemingly unaware that a relatively tiny ~650gr bullet hit it. This bullet did of course cause fatal damage. But there is no way that damage in itself caused an elephant to instantly drop onto it's belly, all legs outward, with no further movement whatsoever.

    Just some thoughts on it. I don't know about jacket integrity on this kind of killing.