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calendar   Thursday - April 07, 2011

Dumb Gun “Knowledge”

I have no idea why this was on CNN’s news page today. It’s a link to a story at Cracked from 10 months ago about the false ideas Hollywood spreads about firearms. As if you didn’t already know. Just for gits and shiggles I guess.

The bit about silencers is good, but it could use a lot more detail. They do mostly tell you the truth: there is no such thing as a silencer. In other parts of the world where such things are legal, even required for hunting in many countries, the devices are called suppressors. Because that’s all they do. They abate a noise loud enough to instantly damage your hearing down to a noise only loud enough to damage your hearing if you’re exposed to it for an extended length of time, like a whole 2 seconds. Guns are loud; wear double ear protection when you go shooting.

There is more to the gun loudness issue than just the boom that the gunpowder makes. Any bullet that travels faster than 1100 feet per second - and most do - is flying faster than the speed of sound, and that means it creates a sonic boom. Even though bullets themselves are relatively small, the noise of their passage is significant. This is why real “silenced” guns shoot slow bullets, around 950fps. And the only way to get any kind of power with a slow bullet is to use a really heavy bullet. Unfortunately heavy bullets at really low velocities aren’t usually fully stabilized, so accuracy suffers. Bottom line is that a truly silent firearm isn’t going to be accurate enough or powerful enough to get the job done at any kind of realistic range. 75 yards, maybe 100, and that’s really pushing the envelope.

Why don’t silencers work? It’s a simple matter of volume. Hur hur hur Drew, good one. No, seriously, it is. Not the “turn down the volume” kind, the “cubic feet of air” kind. To be effective they have to contain all the gas that comes out of the end of the gun and then release it to the atmosphere slowly enough so that there is not pressure wave. That’s the bang sound; it’s the air rushing back in to fill the volume displaced by the expanding powder gases as they leave the end of the barrel. It’s a small thunderclap.

Here’s the math in a simple example. (Sorry Rich, sometimes math is necessary)

The 9mm Parabellum ( 9x19 NATO ) is a very popular cartridge the world over. We’ll use this one for the example. Common ammunition generates about 35,000psi inside the gun barrel’s chamber, and we’ll use a barrel 5” long, which is pretty typical for a full size pistol. Granted that you’ll want to actually use a fully locked breech gun, like the single shot T/C Encore. “silencers” don’t work for jack on revolvers, because they’re open at the back end, but I digress. Stay focused Drew!

Ok, to actually silence a firearm you have to capture all the gas that comes out the end of the gun. Other than using a subsonic bullet, that’s all there is to it.

Typical groove diameter for a 9mm pistol barrel is .355”. With a 5” barrel this means that the volume of the barrel is 3.14159 * (.355/2)2 * 5 = 0.49489 in3. Call it half a cubic inch.

Standard atmospheric pressure is 14.7lb/in2. 35,000 ÷ 14.7 = 2380.952; 2380.952 * 0.49489 = 1178.33. 1178.331/3 = 10.56. This means that the half cubic inch of gas under pressure in the gun is actually 2/3 of a cubic foot at regular air pressure. No wonder it goes bang.

To capture that much gas, you need a vacuum box attached to the end of the barrel. Since no vacuum is perfect, you want to design it a little oversize ... so you’d need an airtight box of about a cubic foot to do the job. You want to handle the heat as well; gunpowder burns at a temperature higher than that needed to melt steel. Good old PV=NRT takes care of most of that; as the gas expands it cools off. But build the vacuum chamber a bit bigger than math requires just to be on the side of certainty. Naturally you’d stick in all those nifty internal baffles to deflect the ejecta blast and to stifle the muzzle flash. And you’d need to figure out the right kind of membrane for both ends that the bullet could pierce without impacting accuracy too much. Maybe Mylar film would work. And you’d need an evacuation valve so you could pump out the air and hold the vacuum. And all of this would be good for exactly ... one shot.


image

a cubic foot baffled vacuum box might actually be silent

design is to scale

Not what I’d call a practical size. And this is for a puny little pistol. A hunting rifle runs at double that pressure so it would require a much bigger chamber; something about the size of a garbage can ought to do it.

(picture of T/C Encore pistol borrowed from The Firearm Blog)

So follow the math and follow the link, and take home today’s lesson: Hollywood feeds you lies about guns. About everything else too, but that’s another lifetime’s worth of posts.

Ok, that’s enough school for one day.

PS - I “cheated” See the first comment and figure out where I went wrong. UPDATED WITH MY ANSWER AND REBUTTAL

See More Below The Fold

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Posted by Drew458   United States  on 04/07/2011 at 08:07 AM   
Filed Under: • Guns and Gun ControlHollywood •  
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