I always wondered: What if your rifle had a barrel that was so long that the gas completely finished expanding before the bullet left the muzzle?
Theoretically speaking, it would be "silenced" but it would not have a silencer.
Anybody know how long the barrel would have to be?
Here is a design that is based on such a concept:
http://www.dillerdesign.com/quietgun/
Shotguns of course operate at much lower pressures, meaning you start closer to where you want to be, but there is also a significant volume of air offsetting that somewhat, and a larger caliber bore for sound waves to exit.
This thing is basically a very long barrel with ports that allow small amounts of gas out along part of the length.
By the time the majority of the gas reaches the muzzle end it has slowed so much that the noise level is very low.
Here is some noise levels of a standard 12 gauge by barrel length:
12 Gauge
28" barrel 151.50dB
26" barrel 156.10dB
18" barrel 161.50dB
The decibel system is logarithmic,
10 decibels higher is 10x more powerful for example,
roughly twice as "loud", and has well over 3x the pressure wave on average.
As you can see
just 10 inches of shotgun barrel alone reduce the decibels by 10.
An 18" barreled shotgun can be twice as "loud" as a 28" barreled shotgun using the same ammunition.
Now consider that the ATF considers any device you add to the firearm that reduces the muzzle report to be a "silencer".
Just barrel can reduce the report by more than that. Even a fake suppressor, those plain tubes that are added for appearance should reduce it by that much. Meaning by their own definition a fake silencer is a silencer.
(But those are widely sold as a cosmetic accessory.)
Obviously a long enough barrel can reduce the report significantly.
The problem with RanB's example is that is assuming you need to get to air room pressure. But you don't need to get to anywhere near that pressure to have something as quiet as a typical silencer.
You can have a report closer to a typical pellet gun at a fraction of the length required to return to normal air pressure.
Though the length is still obviously cumbersome and typically impractical.