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Thread: KE vs OGW vs TKO

  1. #1

    Lightbulb KE vs OGW vs TKO

    No this isn't a thread detailing a three way cage match between some of histories great authors. This is a thread about killing equations.

    I'm going to write them out first and tell you why I think one is better later.

    KE = (1/2 * m * v^2) / the gravitational constant
    where m is mass, v is velocity - if you work it make sure you convert to proper units

    OGW = v^3 * m^2 * 1.5x10^-12
    v and m are the same as above

    TKO = m * v * caliber
    this one also has some fun unit conversion too

    Functionally KE and OGW are going to produce similar results. Granted OGW has the wierd coefficient to make it more intuitive and thus more approachable but they will still render comparable results.

    The issue, in my mind, lies in that velocity is taken to a higher power than mass. All the equations work well when comparing similar calibers but when the velocity difference between the two cartridges is large, velocity is over-represented. Yes, given the same bullet weight the faster cartridge should have more killing power. However, killing power grows disproportionately in these equations. For example, the 44 magnum. Does a 240 grain bullet going 1650fps really have twice the killing power as one going 1300? OGW indicates it does. Kinetic energy indicates that there is half again as much power.

    The kinetic energy equation is actually correct in this respect. I do believe that killing power between the two cartridges is proportionally correct. When comparing the same bullet the KE formula does give proportionally correct numbers with respect to killing power. That doesn't really help us much though. We already know that given identical bullets the faster one will have more killing power. Kineteic energy does not equal killing power though. Kinetic energy would lead you to believe that a 223 is a better killer than a 44.

    While TKO isn't perfect it does give more reasonable numbers when comparing vastly different cartridges. It also takes into account something that neither KE or OGW consider, caliber. The thing that really determines if something dies is the wound cavity.
    Kinetic energy does not wound. Temporary cavity does not wound. The much discussed "shock" of bullet impact is a fable and "knock down" power is a myth. The critical element is penetration. The bullet must pass through the large, blood bearing organs and be of sufficient diameter to promote rapid bleeding. Penetration less than 12 inches is too little, and, in the words of two of the participants in the 1987 Wound Ballistics Workshop, "too little penetration will get you killed."42, 43 Given desirable and reliable penetration, the only way to increase bullet effectiveness is to increase the severity of the wound by increasing the size of hole made by the bullet. Any bullet which will not penetrate through vital organs from less than optimal angles is not acceptable. Of those that will penetrate, the edge is always with the bigger bullet.
    From, Handgun Wounding Factors and Effectiveness, the principles are no different than they are for rifles or game. TKO uses the momentum formula weighted by caliber instead of a biased weighting of velovity or KE to determine killing power. Both KE and OGW ignore any effect that bullet design has. Granted TKO doesn't differentiate between solids and expanding bullets but it is at least addressed. In case you were wondering, yes, Taylor's formula does indicate that a 44 is a better game killer than a 223.

    The super magnum craze that started in the fifties prompted people to use lighter faster bullets "super bullets" to kill game where much slower, larger diameter, heavier bullets were used before. This did give you a much better trajectory and enabled easier bullet placement at the cost of bullet diameter and weight. It take much more KE to do the same job a little momentum will do. If the two methodologies are compared the big heavy and slow bullets will consistently kill larger animals with identical shot placement because the larger, heavier bullet will out penetrate the smaller faster bullet and make a larger wound channel.

    This of course, is mostly my opinion. Feel free to flame!

    Fun reading after the fact:
    Penetration testing with big bore pistols and rifles-

    more of the same-

  2. #2
    Join Date
    Jan 2007
    Spokane, Washington

    Default Wooter's 'L' Factor

    I've always liked John Wooter's "lethality-factor index formula" which was printed in Peterson's Hunting magazine years ago. I put a copy of it in my reloading Thunder Book and use it when comparing calibers. I'm a self-admitted numbers nerd and I feel this formula does a good job of comparing different calibers or different velocity loads of the same diameter bullet.

    Lethal Index = KE*SD*bullet Dia

    338 WM KE(4046)*SD(.281)*Dia(.338) = 384 Lethality-Factor
    308 Win KE(2871)*SD(.248)*Dia(.308) = 219 Lethality-Factor

    Mr. Wooter's went on to say that for dangerous game he would recommend a 'L' factor minimun of 300 which I tend to agree with.



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