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Old 02-21-2010, 11:31   #530
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Join Date: Dec 2005
Location: London
Posts: 5,294
Originally Posted by N/Apower View Post
The point I am making is that there is no way a bullet physically displaces anything beyond the .8" or whatever that it expands to. How could it? If it doesn't touch it, how is it "physically displacing" it?

Ergo, this medium is giving you a false sense of "damage" by measuring part of the temporary cavity. You can see temporary cavity in dyed gel. It's very VERY near the same for .45 and for 357SIG.
You have told us in other posts about the years you have spent in a science degree studying experimental design. I have to wonder what kind of degree this was and what kind of university ran it.

If I were to punch you in the face when you were standing with the back of your head 6 inches from a wall and the back of your head then bounced off the wall, would the back of your head not have been physically displaced by my punch? Yet I have not touched the back of your head and I would no longer be in contact with your face by the time your head hit the wall. I am at a loss to understand what you might mean by "physical displacement" and why you think that the tissue pushed outwards from the bullet track is not physically displaced by the bullet.

You say, "You can see temporary cavity in dyed gel." If you think about it you might realise that the temporary cavity is what it says on the tin. It is not just temporary, it is very temoprary. So if dye has been put into the cavity it has been done after the temporary cavity has collapsed and what you see is not the temporary cavity but the permanent cavity and the infiltration of dye into the tears created by the temporary cavity. You can get an idea of the size of the temporay cavity by high speed photography or high speed moving photography of the bulge in the outside of the gel block as the temporary cavity briefly expands and collapses.

You seem to think that ballistic gelatin is representative of human flesh. This is a remarkably naive concept for a scientist. It has been developed and adjusted to model just one aspect of human flesh. That is, it has been developed to provide approximately the same average resistance as flesh so that the depth of penetration of bullets can be measured in a repeatable way in a consistent test medium to give an approximatley one to one relationship to their penetration in real people. As it does this, it also produces a similar degree of bullet expansion.

If you try to extend the gel model beyond this, it fails. Its tensile strength is not the same and so its resistance to the expansion of the temporary cavity is not the same. The tears is the gelatin block cannot be the same as the tears in real flesh. Even the permanent cavity cannot be the same since part of the size of the permanent cavity in flesh is a result of the bursting of cells and gelatin has no cells to burst so the mode of disruption has to be different.

The size of the temporary cavity is a function of both the rate of displacement needed by the bullet for it to move through the medium at its instantaneous speed and the resistance of the medium to that displacement. That displacement depends on the tensile strength and power of the medium and the mass being displaced. Its tensile strength allows it to tear or resist tearing and its tensile power allows it to act like a spring resisting deformation by storing and then releasing deformational energy.

The greater the mass surrounding the displacement the greater the inertial resistance to deformation. This mass can come from the density of the material of from the size of the block. A bigger block will have greater resistance to the formation of a temporary cavity. Since such resistance will increase the hydrodynamic pressure in front of the bullet, it will slow it faster, expand it further and reduce its penetration. The rate of expansion of the temporary cavity can be such with a high energy bullet that it bursts the gelltin block and so reduces the hydrodynamic pressure and increases the penetration.

It is obvious that ballistic gelatin must have a standardized formulation and consistency. This is made clear by the fact that it has to be used at a very closely controlled temperature. It should also be clear from the above that any testing via ballistic gelatin can be standardized only with a standard size of block but that such a block will work only within a limited window of rates of energy release. That is, a block size and consistency which has been validated on normal service pistol rounds will become progressively less valid for rounds outside that window. In order to validate the Ballistic gelatin block size and composition it had to be tested agains lots of real world results. That was almost certainly 9mm bullet wounds since they were the most readily available from battlefield autopsies. This has significant implications. If ballistic gelatin is approximately valid for 9mm is will become less valid as we move down to .380 and then .32 or up to 357SIG, 10mm, .44 Mag and so on. As we get to .454 Casull and upwards we can think of it as nearly worthless. Ballistic Gelatin is not a gold standard of universal constancy but a rough and ready means of comparing relative bullet performance in the region of 9mm rates of energy transfer.

So, if we look at the Bullet Test Tube, we should look at it in the same cynical manner. We cannot expect it to model flesh and bone and can expect little morre than a guide to some aspects of bullet performance. The thing it gives the greatest comparative result to is the temorary cavity and it does this precisely because it does not collapse back into shape in the way the ballistic gelatine does. (I imagine this is not entirely true since the containing tube will probably collapse it to some extent.)

This then brings us to the issue of the wounding effect relative to the sive of the temporary cavity. It is hard not to be impressed by the size and violence of the temporary cavity shown in ballistic gelatin in slow motion movies though some people seem to manage this feat without difficulty. We can see a similar impressive bulge of tissue caused by bullets shot into dead human or animal bodies and it seems obvious to me, though not to many, that such a bullet wound must incapacitate a considerable volume of tissue outside the permanent cavity to a decreasing extent as the distance from the bullet track increases. I find this a fascinating subject but I have spent enough time on this for now.

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