Everyone is probably familiar with the scene in many a film where someone, usually with a hollow reed in their mouth escapes the bad-guys by swimming away underwater with lead raining all around. The idea is that he is indeed fortunate to have made it through such a precarious situation.
But … I have been told that water offers such resistance that one could easily catch a bullet fired close range from a .357 Magnum, with a bare hand in only three feet of water.
Although I believe it may be possible, I have no desire to test it for myself.
It’s not that water offers resistance, well, maybe it is… The water has to move out of the way of the bullet so that the bullet can pass; water is relatively heavy and attempting to accelerate even a small amount of it a small distance in an incredibly short space of time requires considerable energy. The energy comes from the bullet, when it’s all gone, the force of the bullet is spent.
I don’t know for sure if that would be within three feet or not though, but it sounds fairly reasonable.
Do they not recover bulletsfor test from firing them into 55 gallon drums of water? What 4’ deep or so?
It depends upon the weight and shape of the bullet entering the water. And I would think it would GREATLY depend upon the verticle angle at which the bullet enters.
IANA physicist, but it seems logical that a bullet were fired straight down or nearly so would lose its velocity a lot faster than one fired shallowly. The pressure build up against the bullet accelerates along the vertical component of the flight path.
I don’t quite understand. yojimboguy, could you elaborate?
Water pressure builds up MUCH faster than air pressure as depth increases. You could stroll 3,000 feet down a mountainside with no appreciable change. Most submarines would be squashed like bugs if they tried to submerge 3,000 feet from the surface.
So I’m figuring, as the pressure increases, so does resistance to the movement of an object through it. In other words you’d have to fire your bullet across a much wider trough as opposed to deeper pit.
I don’t think resistance to movement would increase with pressure unless density did too, which, for the case of most liquids is (almost*) not the case.
*(the compressibility oif liquids is not zero, but it’s too small to worry about)
An airplane can go faster at high altitudes than low, I think. Wouln’t the same rules of fluid dynamics apply?
Oh, not the compressibilty factor I guess. Still, I would think a compressibly fluid would do less to slow an object than a non-compressionable one.
Any actual physicists around?
From here
When the .50 caliber bullet was fired vertically downward, the critical distance for complete penetration was found to lie between 4 ft. and 5 ft. Firing at oblique angles of 45 to 60 degrees from the vertical position reduced the lethal bullet travel by approximately 1 ft. When the .30 caliber bullet was fired vertically downward, complete penetration was observed at 1 ft., but not at 2 ft. Based on these observations a person must be submerged at least 5 ft. to feel reasonably safe from .50 caliber machine gun fire and at least 2 ft. for .30 caliber machine gun fire.
Looks like I’m wrong. Damn, I hate when my logic fails me.
Oh well, good thing I don’t make my living doing this stuff.
Here’s a couple of old threads you may find relevent:
Baaum! Shotgun underwater
Bullets in a vacuum
And a link from the first thread:
Underwater firearms
Typical handgun caliber bullets have a very different shape than spitzer point boattail rifle bullets and so have much more drag which increases dramatically in water. I’m actually a little suprised that the performance of .50 caliber is so short underwater becuse the bullet is very low drag and has an awful lot of mass, nearly ten times that of a typical 30 caliber bullet.
It depends on the bullet. Even back in WWII the Germans had a pistol that fired a specially shaped bullet that would travel a pretty fair distance in water. If you can get supercavitation, the bullet continues to “fly” through a pocket of air, reducing the amount of water friction.
A couple years ago I took part in the testing of a new 20mm underwater anti-mine projectile called RAMICS, that had been perfected by a company called C-Tech, which easily and accurately went through 30 feet of water and penetrated armor plate. Standard 20mm ammo was fired for comparison, and they lost all velocity before reaching 6 feet, and yawed as much as 45 degrees within 1 foot of travel. The 30mm version of RAMICS can penetrate over 2" of plate after travelling 100 feet underwater, with incredible accuracy. Hereis a story about RAMICS and the test set up that we used.
Similar results are seen when scaled down to small arms caliber, but there’s not really much point to it.
(1) The angle at which a bullet enters water is of little significance as far as how quickly it will shed its energy. Once the bullet is traveling through water, that’s it – it’s traveling through water.
(2) Water is most definitely compressible, but it seems that it’s not only due to its density and molecular structure. Air and other gases are rightly considered to be liquids, and behave so, under the rules of fluid dynamics.
(3) The “safe depth” will be shallower as the angle of bullet penetration increases from the normal. This is because the bullet, while shedding its energy at a constant rate, will be deflected from the vertical more as the angle increases – AT IMPACT WITH THE WATER SURFACE. Once the bullet is surrounded by water, the trajectory becomes stable and there is no discernible difference in the rate at which it sheds its energy, no matter what the path of travel.
(4) DO NOT try any of this stuff at home!!!
I think you mean air is a fluid. Air at STP is certainly not a liquid, and it’s obviously not true that other gases are liquids.
I don’t know if this is a valid analogy, but can you not think of this as a simple (OK, not so simple) case of refraction of a particle at the interface of two mediums of different densities?
So if the bullet was fired at right angles to the interface, it would travel further, as less of its energy is reflected. as the angle of incidence increases, so does the proportion of energy reflected, until the bullet is at such a great angle of incidence that it bounces off the water.
Just a WAG,
-Oli
From the site :
The second objective was to prove that the super-cavitating projectile could successfully penetrate the mine at that range. In preliminary firing tests, rounds were fired single-shot from a Mann barrel, a test barrel that is standard for determining ammunition accuracy. Once the M197 gatling gun was in place, burst firing was conducted against paper and aluminum targets to measure the total yaw, velocity and in-flight dispersion of the gun’s rounds in a simulated RAMICS engagement scenario. Following that, the mine targets were placed in the tank.
Cool stuff. I’m guessing that the super-cavitating rounds essentially left a bigger and bigger ‘hole’ for other rounds to get to the target.