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Why is exit wound in a rifle gun shot injury bigger than entry wound?

  1. May 28, 2013 #1
    Hi,

    Generally if u see a post mortem skull after a rifle shot injury, exit wound is bigger than entry wound. Shouldn't the bullet lose lots of its energy when coming out, hence smaller wound. Maybe it is spinning or something, but I want to get others views. Thanks :smile:
     
  2. jcsd
  3. May 28, 2013 #2
    Well I'm no expert on ballistics but I think it's actually the other way round , when a bullet or any sharp edged small projectile come in with a high speed it places huge stresses on a very small point on the surface of the target (human body for example) that's why because flesh is usually not the strongest material around it makes a very small hole because penetration at those speeds is easy for the bullet , when coming out the bullet has lost some of it's kinetic energy (speaking about rifle ammo not some pistol one) with lower speeds the stress that the bullet now exerts is smaller on the same surface area.
    It's like puncturing a paper with a sharp pen or something the faster you do it the more chance of making a precise point hole the slower you do it the more the chance of ripping the whole paper sheet apart.Also even though at those speeds it is not common but the bullet can turn and change it's trajectory while between the two sides of the skull after the first initial impact, also it can change it's shape after the impact and the next impact would be with a slower bullet that has a more dull edge rather than a sharp one as before the first impact.
    In other words it has to do with mass/size or shape rather and speed.
     
  4. May 28, 2013 #3

    SteamKing

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    Here is an article discussing the basics:

    http://en.wikipedia.org/wiki/Ballistic_trauma

    The damage caused by a rifle bullet will depend on the weight, caliber, and striking velocity. There will be added damage depending on whether the bullet fragments or mushrooms on impact. Dum-Dum bullets have been banned because they are explicitly designed to mushroom on impact and increase internal injury. Bullets can also tumble inside the would, creating a larger amount of injury than if they passed straight through the body. Small, low velocity bullets may not even have enough energy to pass through the body, especially if they strike bone and ricochet inside the body.

    If a bullet passes through the cranium, the high velocity passage of the bullet will set up a shock wave inside the skull. If the bullet has enough energy on exit, the skull itself may be shattered around the exit.

    Yes, rifle bullets are spin-stabilized, but after striking something hard like bone, their path becomes unpredictable.
     
  5. May 28, 2013 #4

    Danger

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    As SteamKing said...
    Plus... you can never discount secondary projectiles. A lot of the damage of a gunshot wound, including the exit hole, is due to pieces of bone that are broken off and carried along with the bullet. (A perfectly effective bullet, by the bye, doesn't exit the body.)
    I use only non-chemical explosive bullets myself.
     
  6. May 29, 2013 #5

    CWatters

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    I believe the answer is that the bullet creates a shock wave and that shockwave travels outwards much like the waves caused when you throw a stone into a pond.

    In this video the bullet comes out more or less in one piece but deformed. The hole continues to expand after the bullet has passed through..

    http://splodetv.com/video/hollowpoint-vs.-ballistic-gel-0

    Compare with shock wave shape...

    http://goo.gl/WGdfF
     
  7. May 29, 2013 #6

    sophiecentaur

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    I reckon that's got to be one good reason for a large exit hole. There could be a lot more 'stuff' going out than going in. I always wonder about the 'shock wave' explanation and what it really means, bearing in mind that a shock wave is defined as what is generated when an object travels through a medium faster than sound in that medium. Are there many bullets that travel through air at Mach 4?
     
  8. May 30, 2013 #7
    i suspect the bullet has lost its spin and tumbles out


    this is why the beginning of Saving Private Ryan was wrong in the Normandy landings


    it showed soldiers being shot by bullets penetrating the water

    nonsense


    a bullet loses its spin and therfore its stability immediately upon entering water and will start to tumble
     
    Last edited: May 30, 2013
  9. May 30, 2013 #8

    sophiecentaur

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    I seem to remember reading a James Bond book in which they shoot bullets into a tub of water to match the rifling marks in the ballistics lab. They dissipate in not much depth. (I would still poo my pants if I saw bullets coming in my direction, even through the water!!!)
     
  10. May 30, 2013 #9

    CWatters

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    There are plenty of videos showing a bullet going through gell without fragmenting or tumbling but you still get a big hole.

    I'm not clear why some bullets go through blocks of gell without tumbling yet others fragment in water. Myth busters fired various weapons into a swimming pool to work out how deep Mr Bond would have to dive to escape. The answer was just 14" even when the gun is a 50 calib...

     
    Last edited by a moderator: Sep 25, 2014
  11. May 31, 2013 #10
    I think its more about the projo expanding or tumbling. Any decent hunting bullet is designed to expand(or fragment violently if youre after varmints). The big cavity you see in ballistic gel(and human flesh for that matter) is only temporary, as gel/flesh are very flexible. Hence why I doubt the temporary cavity causes the exit to be bigger. You mainly want the bullet to have a nice ballistic efficient so itll hold its velocity better along with performing reliably. In essence, you want the bullet to expand reliably but at the same time penetrate the shoulder bone reliably without fragmenting too much and give a nice big exit for an easily visible blood trail that you can see especially if in the dark(or hopefully no trail at all and for it to drop where its at). Though the main thing is consistency, if you know your favorite bullet fragments on bone, adjust your aim. Thats the main challenge with bullets; consistency. It used to be that bullets with good BC`s(target shooting bullets) and hence good for long range were strictly not meant for hunting and vice-versa. Berger has done a great job in fixing that nowadays though.
     
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