How do Silencers work?

Ok…I did a little research on the web, specifically reading the “How Stuff Works” page, How does a silencer on a gun work?, and while I understand the explination there, it doesn’t fully answer my question.

The main question for me is…doesn’t the explosion of the gunpowder behind the bullet create a lot of the sound? I know from firing M16s on the range that a live round is considerably louder than a blank, which I assume is due to the pressure release that is mentioned in the HSW article. However, blanks, while much quieter than a real round, still make a noticable POP that can be heard from a distance. Why isn’t this sound heard when a silencer is used? I can’t imagine that the sound propagation is entirely directed out the barrel. What’s up?


So long as the bullet’s still in the barrel or the silencer, the sound can’t get past it. When the bullet enters the silencer, there’s a lot of space available within the silencer for the expanding gases to fill. By the time the bullet leaves the silencer (and there is now a place for the sound to leave the gun) the pressure of the expanding gases has dropped drastically. It’s the introduction of high-pressure gases to the atmposphere that makes the noise. The lower the pressure when the gases leave the gun, the less noise.

Hmm. I thought a silencer worked by slowing the velocity of the bullet to below the speed of sound, thereby eliminating the sonic boom. Guess I was wrong.


According to the web site posted in the OP, yer right! You can’t silence the sonic boom with the type of silencer described.


Which is why you use subsonic rounds in a silenced weapon!

Say, when you do use a silencer and a sub-sonic round, do you really get the sound they use in the movies? Ever since I learned how wrong the cop shows are about tracing a call, I’ve been skeptical about everything. (Damn, I wish I knew how to make one of those yellow winking smilies right now.)

      • Yes, they really do work that well. I have heard a commercial unit on a 9mm H&K submachinegun fired, and I’ve seen/heard a few homemade ones over the years. The clank of the action cycling is louder than the thump of the barrel noise. The main mistake TV and movies make is that they show the silencer as being way smaller than it would have to be in order to work so well. That 9mm silencer measured about three inches by twelve inches- it worked great but it was way too big for any pistol. - MC

A couple of other funny movie things are…

I remember on some show (Hawaii 5-0 maybe) seeing a guy with a silencer on a revolver. This doesn’t work for obvious reasons…

On one episode of Nash Bridges, there was a sniper that Nash, et all were after. His weapon? A silenced .340 Weatherby!!! This rifle fires a round that reaches speeds of over 3,000 feet per second. It’s used on large african game mostly, and having fired one personally, i can tell you it just about rips your shoulder off when you shoot.

I can’t seem to find it, but i used to have a picture of a 50 caliber sniper rifle with a silencer on them. The weapon in all is about 8 feet in length and must be fired lying down. The silencer doesn’t actually silence the weapon, but it does take the noise down to that of a 7 mag (!!!)


You may want to check out the replies in a previous thread

This site has a nice cutaway view of a suppressor for a hunting rifle:

Thanks for the replies, but it still doesn’t really answer my main question. (The how stuff works link I found explained most of it.) What about the sound made by the exploding gun powder? Like I said…blanks make lots of sound, but they’re not forcing anything down the tube, and therefore don’t build up any pressure, which is what the HSW link says the sound is from.


The burning of the powder should be mostly contained within the gun. Along with the noise, burning powder is dangerous to human flesh for the obvious heat reasons - that is why it is important for the breech to stay shut for a little while (a fraction of a second) while the pressure of the burning gas drops to safe levels.

In any case, the energy of from the burning powder mostly goes into accelerating the bullet as long as the bullet is still in the barrel. I’m exaggerating somewhat - I’m sure inside the barrel it is quite noisy, but the barrel wall dampens that sound pretty well. The main noise comes when the bullet leaves the barrel and all the powder spills out into the air to burn pell-mell. This is why guns get so much louder when you cut down the barrels - an M16 (with a 20" barrel) is pretty loud, mainly from the sonic boom; a CAR-15 (with a 10 or 11.5" barrels) is freakishly loud, partly from the sonic boom but mostly from all the powder burning out in the open. In other words, if you don’t use the energy to accelerate the bullet, it will become sound energy. According to the 1999 Gun Digest there is a derringer (with a 3" barrel) chambered in .223 Rem. - pretty much the same round that the M16 and the CAR-15 use. Please warn me to leave the county if you want to fire one of those.

Some suppressors, as mentioned above, work by dropping the bullet to subsonic speeds, but many also work just by absorbing the sound of the externally-burning powder. The unused powder just burns up in the spaces between the baffles.

So I think the short answer to your question, Jman, is yes, powder burning does make a lot of noise, but it can be effectively dampened by a few centimeters of metal, whether that metal is the barrel, the chamber wall, or suppressor baffles. A good suppressor can make the noise of a .45 comparable to that of a .22 (or the noise of a .22 comparable to that of a twanging bowstring, so I’ve read).

I am no expert but I don’t like part of this answer.

Burning powder doesn’t make squat for noise unless it is in a confined space. A pile of it just goes woosh.

The gases from a blank round DO force something down the tube. They are pushing on the gas that is already present and which has to be forced out of the barrel for the new gas to take its place. The gas resists being moved a LOT less than a bullet, and blank cartridges use less propellant, so the “bang” isn’t nearly as loud.

Also, you mentioned firing M16’s. In order to properly operate when firing blanks, the M16 requires a BFA (Blank Firing Adapter) over the flash hider. This also helps to deaden the percieved sound.

The weapons that you mentioned as being louder than a weapon with a standard-length bbl are generally because of the addition of a muzzle brake. These divert some of the pressure and sound out the sides and to the rear of the bbl’s muzzle, so the percieved sound is MUCH louder for anyone not standing downrange of the weapon.

I can personally vouch for this, as someone who has taken a blank round to the side of the head at close range whilst on manoeuvres many years ago. Blanks push out an awful lot of hot gas (and the one that got me had 2 feet of muzzle flash as well), and although the propellant is less than a live round, it is more than enough to kill you if it goes off directly at your head. Fortunately, it was tangential not straight at me, or I wouldn’t be typing this now.

The best explanation I ever heard about silencers (some repeats here) was in the New Scientist The Last Word

Afraid I have to disagree with your explanation, Boris B. Unconfined burning powder makes litle noise, as you can verify by igniting a small heap of it (be careful of those eyebrows, now!) You get a sort of roaring “whoosh!” but nothing like the report of a gun.

The main source of noise is the sudden release of high pressure when the bullet exits the barrel and allows the propellant gas to escape. Same as bursting a balloon, or doing that thing with dry ice in a plastic bottle.

In a firearm, the maximum pressure is reached almost instantaneously and then falls as the bullet travels down the barrel. That’s why shorter barrels give louder reports - the pressure is higher when the bullet leaves the barrel.

The simplest silencer is just an expansion chamber, like the infamous “soda bottle taped onto a .22”. The released propellant gas expands into the bottle, losing much of its pressure before it can expand freely in the air and make a loud report. Other silencer designs use wire wool to rapidly cool the gas (antiquated, dangerous), a sequence of baffles to control the expansion further (most common design - see the link I posted above) and sometimes contain water or grease to cool the gas by vapourisation (“wet” silencers, rare, reputedly very effective.)

Wow, multiple simulpost!

Yeah, I know about blank adapters…unfortunately, they still don’t build up enough pressure to eject the round successfully about 20% of the time. :mad: Thanks for the responses everyone…sounds good to me.


The BFA allows the gas to escape to the atmosphere, but across an area SMALLER than that of the barrel alone, so the retort would be LOUDER for the same amount of powder. The BFA merely diverts a higher percentage of the gas back through the gas tube to cycle the bolt back, since the round usually provides that blockage, however brief. Try it next time you blow caps, fire with and without the BFA, but do not point it at bwanasimba.

And it’s a flash suppressor. A flash hider is a long trenchcoat. Trust me on this one.