A silencer's or suppressor
's job is to reduce the compression wave
created by the firing a bullet. This compression wave follows directly after the bullet
, created by the extremely high pressure from the cartridge
exploding. But the pressure itself has nothing to do with the sound. The change in pressure creates the wave. In a typical gun, two major waves will exit the gun immediately before and after the bullet. The first is created by the air in front of the bullet that is now being pushed out of the barrel by the bullet. The second is created by the hot, expanding gas created by the explosion
. There are reflect
ed waves after this one, but those are the major two wave
s. A silencer does not "let the pressure out slowly," it dampens the attack of the wave.
It would be nearly impossible to slow down a 3,000psi burst of air exiting a barrel of a fixed length, and in fact, you want it to exit as quickly as possible for all purposes. You simply want the the attack of the wave (the part before the crest) to be longer; you want to smooth the wave out. Most suppressors do this by reflecting the air within the suppressor in either one large chamber or several small chambers. Then the compression wave will be reflected back onto itself, creating a lower amplitude wave with a longer attack and decay.
Further, most car mufflers are not made like suppressors. Motorcycle mufflers and high performance mufflers are somewhat like them, as they are of a straight-through design, but they use pipe of a different diameter which helps dampen the sound (along with various other differences). A suppressor is the same diameter as the gun barrel for obvious reasons. Most car mufflers use a series of baffles to suppress the sound, and they are designed for a completely different environment.