How a gun silencer work?
In: Physics
A gun works this way: The primer ignites the powder. The powder then burns, producing gases with increasing pressure. When pressure is enough, it dislodges the bullet from the case and starts pushing it down the barrel.
From here, we will consider the .223 Remington. The pressure in the chamber can hit about 55,000 psi, extremely high. Given the extra volume of the barrel, this pressure decreases by the time the bullet reaches the end of the barrel. But it’s still maybe 15,000 psi when the bullet leaves the barrel. There’s a lot of “it depends” in this, but we’ll go with this number.
So you have the end of the barrel, all the ambient 15 psi air around it. Then a bullet exits, and suddenly a volume of very hot 15,000 psi gas slams into the air and rapidly expands since it is no longer constrained. This makes the air have to get out of the way really fast, and boom, shockwave, same as from an explosion.
In general (depends on design), a suppressor is a tube with a bunch of small chambers in it, with dividers between those chambers that have a hole for the bullet to pass through. The gas expands to fill the first chamber, then the second, and so on before it hits the atmosphere. This extra time also allows the gases to cool more before exiting. To give you an idea of how much cooling, a suppressor can get hot enough that you may get a doctor visit level of burn if you dump a full magazine through one and then grab it (people who use them a lot often bring oven mitts or welding gloves). Shoot even more, and they can start glowing.
Due to this chance to expand and cool within the gun, the gas pressure and temperature is much lower when it hits the atmosphere, so the boom isn’t so bad. It’s still loud, just not quite as ear-splitting loud. Normally a suppressor can take about 30 dB off a gun, so we go from instant permanent hearing damage loud to needing to experience many shots to get hearing damage.
We can do more to get this quieter. Sometimes you’ll have wipes between the chambers, which are solid rubber discs the bullet has to punch through, and they mostly seal up behind the bullet, trapping the gases (they only last a few shots though). Instead of this more modern designs create turbulence to interfere with the gases leaving the barrel, kind of keeping them trapped longer by making them roll around. We can also add to the expansion volume by shrouding the whole barrel with a suppressor, allowing the gases to fill the entire area around the barrel counting as one big expansion chamber.
Suppressors do not stop the sonic boom of supersonic bullets, the “crack.” They also do not quiet the sound of the moving parts of semi-auto guns (which can be kind of loud).
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