Gun silencers - What they do and how they do it.

Why does a gun make noise?

To propel a projectile from a handgun, rifle, or basically any other firearm, a highly explosive material is used. In theory, the more pressure you can put behind a bullet, the faster it will fly.

If you have read my explosion write-up, you know that the sound from explosions is caused by the rapid expansion of gases. This rapid expansion is what pushes the bullet ahead, so logic should leade us to believe that there isn't such a thing as a silent gun


In theory, if you set of a very very small explosion in a gun barrel that was closed (in other words, in a very strong piece of pipe), you wouldn't hear the explosion too well. Why? Because the shock wave from the expanding gas never reaches the air outside. Although the pressure inside the barrel might be very high - if the pressure isn't enough to break the barrel - you don't hear a thing.

The thing is - trying to fire a bullet from a closed pipe is rather useless (we want the bullet to fly, right).

Think about a balloon. If you pop the balloon (letting all the pressure out at the same time), you get a bang. If you were to untie the balloon, and let the air out, you would hear it, but chances of anyone in the next room to hear it is rather limited.

The theory behind a silencer then, is to "let the pressure out of the gun gradually".

How a muffler works

Inside your car, you have an engine. Inside this engine, there can be anything from a few hundred to a few thousands explosions per minute (when the engine is running, obviously). Hence internal combustion engine. If you ever have driven around in a car without a muffler, you know how much noise that makes.

A muffler works like this: The gases that come out of your engine after the gasoline has been mixed with oxygen and ignited, have very high pressure. This pressure is a good thing, because that's what makes your engine turn and your car go forward. Anyway - the pressure goes into your exhaust system. This system goes into your exhaust muffler. The muffler is, essentially, a form of buffer. Put crudely, your exhaust system is a pipe leading to a chamber, with a pipe leading from that chamber out in open air.

Inside this chamber, the high-pressured gas has the possibility to expland, causing a loud sound. But because the chamber is contained, the sound can't go anywhere - The pressurecontinues out of your tailpipe, and that was the story of that explosion. Although no cars are soundless, there is definitely a difference between a car and a gunshot.

Now for the fun part - how a silencer works

The pressure behind a bullet is immense - about 2 500 PSI is not unusual. When this amount of pressure is released into the air, you hear a loud bang.

When you affix a silencer to your gun, the exact same thing happens as with the muffler in your car. Inside the silencer, there is a series of chambers that lets the pressure expand, before being led out of the barrel, drastically decreasing the sound.

To make a gun soundless, though, there is one more factor than the pressure from the actual explosion - namely the speed of the bullet. If the bullet goes faster than the speed of sound, there will be a sonic boom. To avoid this, the bullet will have to travel slower than the speed of sound (about 760 mph / 1200 kph). Because of this requirement, silenced weapons have to use subsonic ammunition to be truly silenced. Because a "normal" bullet can happily fly along at 900 mph, the subsonic requirement causes the guns to be less effective.


killermonkeys' wu on this topic is factually more correct than mine - I fell victim of over-simplifying, thereby adding factual errors to this node.