If high speed bullets carry a destructive shock wave, why no paper target damage?

[mbs357]

So how loud is the bullet's sonic boom?
That is, what noise would a gun/bullet make if there was absolutely no report--just the bullet's boom?

 

[brickeyee]

"density inside the wave is typically triple the normal density"

Not the correct definition of a shock wave. A shock wave is when the particel velocity exceeds the speed of sound in the media is a more workable definition, but still not entirely correct. See below.

"A direct hit from a nuclear bomb only creates about 5000 psi of pressure.

So I think it's safe to say that high explosives do not cause shock waves in water, they cause pressure waves."

I would suggest you review "Explosives Engineering" by Cooper.
Sound velocity is constant in the elastic region of a material. Up to the elastic limit you have a pressure wave. When the elastic limit is exceeded the wave front starts to speed up (is no longer proportional to change in pressure) and the pressure wave becomes essentially a vertical front. This is the transition to a shock wave. A wave is said to ‘shock up’ since the speed now increases with increasing pressure. A plot in the Hugoniot plane shows the pressure and velocity required to move into shock wave behavior.
See Cooper Fig 18.4.
TNT can produce pressures of 5800 PSI (40 MPa).
Cyclotol 8700 PSI (60 MPa).
Even more powerful explosives are available.

 

[Kurush]

Not the correct definition of a shock wave. A shock wave is when the particel velocity exceeds the speed of sound in the media is a more workable definition, but still not entirely correct. See below.

I wasn't giving a definition there, I had defined it earlier. Your definition is just an atomic description of the definition I gave earlier.

What I was saying above is that you must actually compress the medium in order to have a shock wave, so I made a "napkin" calculation of the pressure necessary to effect sufficient compression of water.

When the elastic limit is exceeded the wave front starts to speed up (is no longer proportional to change in pressure) and the pressure wave becomes essentially a vertical front.

You have just given a correct description of the formation of a shock wave in solid material. In a solid, the transition to shock occurs when the elastic limit is exceeded, in a liquid it occurs when the compression is sufficient to cause a phase change.

Obviously, fluids have no tensile strength and therefore no elastic limit so the definition for solids can't even be applied to water. If you want to expand on my "napkin" calculation be my guest, I freely admit it is not fully correct, but I do believe it is a valid lower bound.

 

[mbs357]

So, particles moving past the speed of sound = shock wave? Particles as in dust particles/molecules/etc? What do you mean by particles?

And when people usually refer to a shock wave, like an explosion's shock wave, they're actually talking about a pressure wave?

 

[Too Many Choices!?]

We don't need no stinkin' scientists to answer this question....

We just need,"Too Many Choices!?", and a few words: Here we go,AHEM, Paper ain't people, therefore it doesn't compress, therefore it does not create a preassure wave.The papers rigidity(or elasticity depending on material) is overcome before the paper cab can compress to give the explosive effect of the preassure wave release upon exit. No Compression, no explosive wave. Add more paper, and wet it like another poster said. .40 Cal hollow points out of a, Glock 23, do a number on a phonebook, but don't penetrate, and leave one hell of a mess of the paper intwined(literally welded together) with the expanded bullet;"Shrapnel". I don't want to think about it mangling flesh that way

 

[Daniel T]

All the scientists came out for this one.

What's your point? Would you rather end the discussion at "BULLET GOES BANG!"?

If it's over your head, pass it by.

 

[bad LT]

Agggh - You guys are reminding me of my fluid mechanics tests.

The Horror, the horror

 

[Justin]

So how loud is the bullet's sonic boom?
That is, what noise would a gun/bullet make if there was absolutely no report--just the bullet's boom?

Again, my experience with pulling targets in the pits at a High Power rifle match.
You hear the bullet fly overhead, which, with hearing protection in, sounded roughly equivalent to the report of a .22 pistol.

Then shortly after you hear the report of the gun itself.

So the whole thing sounds like a sharp *crack* overhead followed by a distant *boom.*

 

[Kurush]

So, particles moving past the speed of sound = shock wave? Particles as in dust particles/molecules/etc? What do you mean by particles?

In air and water we are talking about molecules.

And when people usually refer to a shock wave, like an explosion's shock wave, they're actually talking about a pressure wave?

In my experience most people will refer to overpressure as a "shock wave", yeah. However HEs do cause shock waves in air and in critters, out to a certain range.

If someone says: "wow, look at the shock wave of that bomb" referring to the visible shock front, he is using it correctly.

If someone says: "we dropped a depth charge, and the shock wave made a huge water spout" they mean "pressure wave".

If someone says: "The deer I shot had bruising from the big shock wave that went through it" he means "pressure wave".

Finally, if someone says: "I shot a deer with my Hydro-shoks in the ankle and the 'hydrostatic shock' sucked its brains out!" they are full of baloney

So how loud is the bullet's sonic boom?

Not very loud, the bullet doesn't displace much air so the amplitude of the shock wave is pretty weak. Most of the noise from a gunshot comes from air being displaced by the expanding gasses produced by the gunpowder. I'm not volunteering to have a .50 shot right past my ear mind you but I don't think it would break your eardrum.

 

[brickeyee]

"...in a liquid it occurs when the compression is sufficient to cause a phase change."

The 'elastic limit' in water is normally taken as cavitation.
No problem. How big a bubble do you want?

 

[Cosmoline]

What I have read in many sources is that once bullets get beyond about 2,200 fps they start to have an "explosive" impact on flesh from their speed alone. However I have never seen a really scientific test of this. Ballistic gel isn't going to work, unless it can replicate the myriad of tiny capilaries and connective tissue that is supposed to bear the brunt of the shockwave.

 

[RyanM]

What I have read in many sources is that once bullets get beyond about 2,200 fps they start to have an "explosive" impact on flesh from their speed alone.

Partially true. Somewhere in the 2,000 to 2,500 fps range is a threshold at which standard construction softpoints start to fragment. The combination of fragmentation and relatively large temporary cavity have a synergistic effect; the fragments make multiple small holes, then the temporary cavity is able to tear the tissue much more easily.

For instance, if you compare this picture with this one, you can see that the maximum diameter of the temporary cavities is about the same (a bit under 8" for the FMJ, around 9" for the softpoint), but the fragmenting bullet makes a much larger permanent cavity. You can also see that while this .30-30 at 2017 fps created quite a large temporary cavity (a little more than 6") and did fragment a bit, there just weren't enough fragments to cause an "explosive" effect like the .308.

Testing in actual living tissue has borne this difference out. Click here, and download the article entitled "Bullet Fragmentation: A Major Cause of Tissue Disruption" (third one down). On the third page, you can see photos of cross-sections of thigh tissue of a piggie. It may be a bit hard to tell, but that tiny little white spot in each of the three top pieces of meat is all the bullet hole you get with nonfragmenting, non-expanding ammo.