AK-47 Underwater at 27,450 FPS

In the past we have written about awesome firearms that were used underwater by the elite military Special Forces.

But good thing many of us Joe average are always fiercely curious. Ever thought of of what happens when you shoot an AK-47 under water?

This video shows that while it is possible to shoot a gun under water, but is advised not to do so.

The slow motion shows a great deal of hot gases pluming into the water. You can learn a lot from this one. The oxygen needed to launch the bullet is contained within the bullet, so when firing under water, this sealed cartridge with primer, propellant and bullet work as they are made to work on dry land.

The difference is water has more density, so the power of the shot bullet is greatly reduced but still could be lethal at close range. See the rest of the AK-47 Underwater at 27,450 fps footage here.

The bullet is a projectile...

Yeah...I know...and in retrospect I realized that was probably what the "sigh" was for.

Shoddy journalism abounds, particularly when the subject is firearms and attendant items.

But, as I said, explanation of this "process" is difficult for the non-shooters or anyone without a good understanding of the entire process of what makes "bang" go "bang".

Regrettably, I have found the largest portion of stupid writing comes from the pens of so-called "gunwriters".

#1, Any firearm cartridge will fire just fine underwater (so long as the powder remains dry) or even in a vacuum because all smokeless gunpowders contain an oxidant. It needs to because the force of the explosion (actually, it’s “conflagration”) would drive driving most of the air away from the point of ignition so very little “new” oxygen would be able to get to the source of the fire. Buzz could have shot Neil dead with a plain old .38 revolver when they were walking on the moon. And what air might be left inside the cartridge would be wholly inadequate to supporting that large a fire.

#2, That’s an AK-74, not an AK-47. Dusty (in the video) only calls it an “AK” and the ASJ apparently can’t tell the difference. Without getting into nuanced details like muzzle devices (which are user-changeable), the appearance of the gas blocks on the two are distinctly different. The gas block on the AK-47 appears sloping or angular but on the AK-74 it appears distinctly perpendicular to the barrel.

I’m surprised the extra back pressure didn’t cause it to blow up. It effectively increased the weight of the projectile by quite a bit.

Blow-by is running point for the bullet.

There's always some propellant gas in the barrel moving ahead of the bullet. All cartridges (except some specialized target shooting rounds) are loaded so the bullet has to move a short distance after it is dislodged from the cartridge casing before it meets the barrel's forcing cone and begins being driven onto the lands. But because there's already so much pressure inside the casing, just the few hundred PSI needed to break the bullet free, some propellant gas rushes past the bullet before it begins to "obturate" or seal the barrel.

Even after obturation the "seal" is not perfect and as pressure behind the bullet rises, propellant gasses continue to "blow by" or sneak past, particularly down in the corners of the grooves.

So once the firing sequence begins, a bubble of gas gets in front of the bullet and remains there until the bullet has exited the muzzle. So it's not the bullet pushing aside the water in the barrel, it's the blow-by doing it. The amount of pressure cooped up behind the bullet isn't much different from when it's fired in air.

Watch the video closely and you'll see gas issuing from the muzzle for several frames before the bullet emerges. Even that doesn't happen until it's outrun the muzzle blast, which in this case appears to extend the better part of a foot past the muzzle brake. So the bulett already is a foot or so downrange before it runs into water again.

“Quite simply, the oxygen IS in the cartridge, for the easiest understanding for most people.”

Yes, the oxygen is in the cartridge, not “contained within the bullet” as the author wrote.

I know that in everyday conversation people say “I’m going to buy some bullets” or “I loaded a bullet into the chamber” when they really mean “cartridge(s)” in both cases, but I believe a published article in something that calls itself a Shooting Journal should have done better.

Or maybe I’ve just lost track of the changes in the language. Seems like a number of words have changed meaning since I first learned them.

Sigh.

Not at all. Water. like air, surrounds the bullet, so there is no “impact” just equalized support from the media- in this case, water not air.

I have fired HST 40 cal , 9mm and 45 acp from several handguns under water with not expansion at all- not enough velocity upon impact nor with the media it is in. Guns cycled normally, fired several repeat shots w/o issue.

ARs under water too- no bullet issues nor much efficiency- water is a great inhibitor.

No signs of high pressure- normal function again through several rounds of commercial, handload and military rounds, BTHPs, Soft point and FMJ.

Shotguns are rather sad underwater-shark guns are contact weapons- so there is not water to contend with....

Regards

Just for a moment thought the article was about Combustion Light Gas Guns scaled down to hand held size. The CLGG is the forerunner technology before the Navy switched track to pursue the electric rail-gun.

The technology demonstrator started with a 16mm round, went to 45mm, and finally 155mm. 4km second or about 13,000fps velocities were achieved. Inconsistent ignition of the propellant charge, thus inconsistent chamber pressure presented as a problem for the 155mm scale prototype. Hitting a target at 200 mile range with a dumb round wasn’t possible with this condition.

http://www.dtic.mil/dtic/tr/fulltext/u2/a462130.pdf

https://www.aa.washington.edu/files/research/ramaccel/publications/STA-witherspoonOct2000.pdf

"Seems to me that the blow-by would then be greatly compressed thus putting additional resistance on the bullet. The bullet may even go past the blow-by and touch the water. No different than a plugged up barrel...."

It's compressed but it's compressed in a specific direction. The blow-by has momentum. Air has mass and everything that has mass, when in motion, also has momentum. And Newton's first law says that everything in motion starts out trying to move in a straight line. Which in this case is >>>> toward the muzzle. The reason the propellant gas isn't backing up against the bullet is that it's in the nature of all moving objects to continue moving in the same direction. It was moving toward the muzzle when it escaped past the bullet and it will continue moving in that same direction unless and until something changes its mind.

By the time the bullet is fully obturated, there's in the vicinity of 10,000 PSI chamber pressure behind it. The volume of the water in an 18" 5.45mm barrel is less than three-quarters of a cubic inch, less than half an ounce. That little timy bit of water is just too minuscule to change the intentions of a gas driven by 10,000 PSI.

Before the bullet ever strikes the lands, there's already a rush of several hundred to a few thousand PSI of hot propellant gas entering the throat of the barrel (the gas always tries to outrun the bullet, otherwise it wouldn't be much of a propellant). If there's already water there, and since water is (virtually) incompressible, something has to give. Air and water can't both occupy the same space at the same time. So the highly pressurized air drives out the water, and there's only one direction for it to move in: toward the muzzle.

At which point the water also has momentum >>>> toward the muzzle. All the propellant gas needs to do from that point forward is keep the water accelerating in that direction and the bullet will remain essentially dry until it's clear of the bubble created by the muzzle blast. And there's no arguing but that the first thing to reach the muzzle in the video linked in the OP is not the bullet but gas.

You also can see strong evidence of my claim of gas having momentum when it exits the muzzle. At first a little of the gas escapes through the ports on the sides of the muzzle brake but that's before any of it has got as far as the muzzle. The overwhelming majority of gas continues in a straight line. Even after it leaves the confines of the barrel, the gas still mostly travels in a straight line despite the considerable resistance exerted by the water it's colliding with. If it were being overwhelmed by the resistance from the water, it should have "bloomed" to form a more spherical shape. But even outside the confines of the barrel, it's still pretty pretty linear, sort of carrot shaped. So momentum is still the primary influence on its direction of travel, even once it's left the barrel.

If it's no different from a plugged-up barrel then the video obviously must have been faked because the rifle should have blown up.

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