Part II
So far all of this is well established physics and really all I have done is repost the work of others.
But now, if I may, I would like to go a step further than most and discuss the biological importance of exit wounds when it comes to killing people.
First its important to establish that I do not believe in over-pentration. Its synonymous with over-effectivness as far as I am concerned. The FBI isn't too keen on it either:
Quote:
An issue that must be addressed is the fear of over penetration widely expressed on the part of law enforcement. The concern that a bullet would pass through the body of a subject and injure an innocent bystander is clearly exaggerated. Any review of law enforcement shootings will reveal that the great majority of shots fired by officers do not hit any subjects at all. It should be obvious that the relatively few shots that do hit a subject are not somehow more dangerous to bystanders than the shots that miss the subject entirely.
Okay, so to understand where I am coming from on the next part you have to know that I am an EMT and have responded to many trauma cases over the past nine years, fatal and otherwise A handful have even been gunshots so although I am not a battlefield surgeon I do have first hand medical training and experience with trauma victims.
If we assume that bleeding to the point where blood pressure falls and victim loses consciousness is the most realistic and reliable method of stopping a human target (head and spine shots of course are always instantly stopping but we can't assume we will always hit them) then it all comes down to blood loss.
When you speak of blood pressure medically you are speaking of the three vascular 'containers' Inter vascular, fluid inside the veins and arteries, intercellular, fluid inside the cells themselves, and interstitial fluid, fluid in-between the cells.
Except for the inside of the lungs and digestive tract ("hollow" organs), there is no empty space inside a human body. What isn't cellular tissue, bones, arteries etc. is filled with interstitial fluid, usually just a thin film that lubricates the outside of all the tissues and organs so they can move around without damage. This fluid is under mild pressure at all times.
When you rupture a blood vessel internally the blood immediately starts flowing out into the interstitial space. We call this "third spacing" Because the vascular pressure is higher than the intercellular pressure the blood flows out rather than the interstitial fluid flowing in.
But, the human body is prepared for this. Since there is no empty space for the blood to flow to as you bleed internally the interstitial pressure rises as your blood pressure decreases. Various membrane 'firewalls' divid the interstitial area into compartments of different sizes like water tight doors on a ship so that internal bleeding does not have to raise the pressure in the entire body, just the section that is breeched.
Now, what this all means is that as you bleed internally that blood doesn't just go nowhere and and start filling up your feet, its trapped by hydraulic back pressure and compartment membranes, and when blood stops flowing it starts to clot almost immediately. Most people never get to see how blood clots on a large scale but its pretty amazing, an internal blood clot in a healthy person quickly becomes as hard as rubber and can be very large, the size of a softball or greater.
This blood clot provides a semi-regid structure around the hemorrhage and applies back pressure to the ruptured artery. If allowed to form (not washed away by rapidly flowing blood) this clot will seal the wound both internally and external and stop this third spacing of blood and stabilize blood pressure.
This is why applying pressure to even a deep wound usually works, its stops or slows down external flow of blood and forces it to back up in the tissue around the wound and clot. The effect is called 'tamponade'
Now, wether this works or not depends on the rate of blood flow and the area where the bleeding is occurring, some places, like your abdomen can accommodate a great deal of blood before tamponade, while arteries embedded in muscle tissue, such as your thigh, are very effectively controlled by tamponade.
The important thing ballistically though is that in order to defeat tamponade you have to give the blood somewhere to go, that is, out of the body. Internal bleeding is important but its also important that it has somewhere to go so it can keep moving and keep washing out the clots.
This is all just a very detailed way of saying that in order to make someone bleed to death you need to put holes in them for the blood to flow out of and probably why the number of handgun wounds sustained is the single greatest predictor of mortality rather than placement or caliber.
This means the more holes, the better. Exit wounds are like shooting them again without having to expend another bullet, not to mention the fact that due to the differences between the compressive puncture wound in skin and the tearing exit wound even FMJ bullets make larger exit wounds than entrance wounds.
Now given bones, skin, tendon etc, even an FMJ bullet may fail to fully penetrate a target but it has a better chance than a bullet with less penetration. I believe that its only bullets which can be expected to reliably create exit wounds which should be tailored for controlled expansion to give the very slight edge provided by larger bullets but only if it does not prevent full penetration.
For 9-40-45 calibers there isn't enough energy to do this and going with SWC's is about the best you can hope for when it comes to increasing permanent cavity without preventing full penetration.
Originally Posted by breakingtime91
