Ballistics question, re: hydrostatic shock, kinetic energy


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Nightcrawler
April 14, 2003, 04:34 PM
Hydrostatic shock is, as I understand it, the shockwave in your body (which is mostly water) from the impact of a bullet. A great deal of energy is required to create this, hence it's usually only seen in rifle rounds.

Hydrostatic shock is, as I understand it, basically the same thing as when a small meteor hits the earth and leaves a big crater. The more mass and the more velocity your projectile has, the more energy it creates when it hits something.

It's why a shot from a .30-06 FMJ round is more devastaing than a .357 magnum round, even though the .357 is a larger (and in some loadings, heavier) projectile. The .30-06 is going almost twice as fast.

Am I correct in this? That's my question. I've been trying to explain to someone that knows nothing about ballistics why rifle rounds do so much damage, as opposed to just leaving a (say) .30 caliber hole in one side and out the other. IF that were all they did, after all, .45ACP would be MUCH more lethal, as it'd leave a much bigger hole clean through. It's also why even FMJ rounds can leave a bigger exit wound than an entry wound, if they're going fast enough.

It's all about the kinetic energy, right? Meteor hitting the earth?

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benEzra
April 14, 2003, 06:41 PM
The easiest answer (though some would argue oversimplified) is simple kinetic energy. A .22 long rifle 40-gr HP at, say, 1100 fps has (I'm working from memory here) around 180 ft-lb of energy. A 40-gr HP out of a .223 Remington traveling at 3650 fps has around 1200 ft-lb of energy. Assuming both bullets stop in the target, the .223 delivers 20 times the energy.

Unlike simple momentum, kinetic energy (i.e., the ability to do work) is proportional to the velocity SQUARED. So if you triple the velocity, you get nine times the kinetic energy, etc. This is why rifles carry so much more energy.

There is a lot of talk about "hydrostatic shock," "shock waves," etc. in tissue. True SHOCK waves DON'T occur in tissue or gelatin, as by definition a shock wave involves something that is supersonic in that medium, and I believe even a 4000-fps varmint bullet is SUBsonic in tissue and gelatin (i.e., slower than the local speed of sound). I believe "shock" in the sense it is often used refers more to a physiological effect than any sort of physical phenomenon.

What you do have is a PRESSURE wave that originates as the gelatin is thrown aside and backward at hundreds or thousands of FPS by the bullet, producing the temporary cavity. I am under the impression that some authorities (such as Fackler et al) discount the effects of temporary cavity unless the velocity is higher than a magic number (2700 ft-sec?). I am persuaded that the effect is more of a continuum, however--basically, wherever the amount of stretch (or compression of tissues on the other side of the stretch) exceeds the elasticity of tissue at the microscopic scale, you have tissue damage at that point.

Stevie-Ray
April 14, 2003, 07:12 PM
M...kay. It's clear to me now.:banghead:

Standing Wolf
April 14, 2003, 08:24 PM
Hello, benEzra!

Thank you! I've more or less understood most of that for years, but have never seen it all in one place at one time. Now it actually makes sense.

Jim March
April 14, 2003, 08:46 PM
Yes, there's some sort of extra tissue-destroying "shock wave" or something at a certain velocity. Reports differ on where it is - some reports have it happening at speeds as low as 2,000fps if the bullet width and shape is right. A 300grain flat-front .45 slug out of a 45-70 rifle, for example, seems to produce "extra shock effects" at somewhere between 2,000 and 2,400fps.

A needle-pointed spitzer bullet may need a bit more speed to do the same thing? I don't know for sure, but the differences in both caliber and bullet shape would explain why reports on exactly where this "speed effect" is in FPS vary so greatly.

Art Eatman
April 15, 2003, 08:45 AM
Photographs have shown the "shock wave" or pressure wave when a bullet hits a test medium such as gelatin. Simplest put, the pressure forces the medium away from the path of the bullet. The size of the permanent cavity or the amount of recovery varies with both the material and the amount of engergy causing the pressure. Kinetic energy provides a measure of the available energy to create the pressure to do the damage.

A slow-moving bullet which does not expand will not create as large a cavity as a higher-speed expanding bullet. If the bullet does not tumble, we can have the proverbial "30-caliber hole". A tumbling bullet will cause a wound channel much like a small knife.

With high-speed, expanding bullets, there is a tremendous amound of damage to flesh. Even if the wound cavity collapses back to some small diameter, tissue is destroyed or damaged well outside the path of the bullet.

I have seen a 100-grain bullet from an '06 dismember a jackrabbit into three separate pieces. I have seen a 150-grain bullet leave a four-inch diameter path of tissue destruction/damage in a deer's ham. The larger, slow-moving bullets don't do this sort of damage.

Whether or not a bullet expands depends on its design. Most hunting bullets are designed for velocities in the vicinity of 2,600 to 3,300 feet per second. Too fast, and they'll expand too soon and possibly disintegrate. Too slow, and they expand very little. That's why the bullet must be matched to the expected game animal, the probable distance of a shot, and the particular powder loading.

Hope all this babble is helpful...

Art

Bonker
April 15, 2003, 11:21 AM
Forget the meteor example.

Think of hydrostatic shock as the wake made by a boat in the water.

If the boat is going fast enough you'll see a very distinct "V" shape starting at the front and continuing well behind the boat.

The faster the boat (bullet) is going, the more powerful and disruptive the "V" will be.

Now imagine a 3d version that a submarine might make where the "V" would be shapped like a cone. That's what happens with a fast bullet in a mostly liquid human (or animal) body.

That cone of super-compressed fluid (yes I know you can't compress a liquid, work with me here people :) ) can do as much damage as steel.

The faster the bullet, the bigger the cone which is why a .223 can make an exit wound as big or bigger than a 12 ga. slug. A bigger bullet will also make a bigger cone and exit wound but not nearly to the degree that a fast one will.

Go to the lake and watch a huge yacht moving slowly in the water. It leaves only a small wake although it is a little more than a small boat at the same speed. Now watch a small speedboat whiz by. The wake is HUGE compared to the yacht right?

Now put jets on the yacht and speed it up to about 100mph and you have an example of a 50BMG :)

Double Naught Spy
April 16, 2003, 10:40 AM
The V example is interesting, but don't forget that when one object strikes another, there is force that is transferred that radiates in the direction of travel. For example, we have all seen BB shots on plate glass windows. The BB impacts and the energy wave radiates forward and out from the point of impact. This is called the 'cone of percussion' by people who study fracture dynamics. The size and shape of this forwardly projected cone will depend on the medium struck and what struck it (size, shape, weight, mass, velocity).

If you followed the V example given by Bonker, you might be inclined to believe that the wound cavity should be largest at the entry of entry and smallest at the exit as would be expected by a boat's wake. In fact, the opposite the case.

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