They are both legal and illegal.
They are legal on airguns, however anything that can be used on a firearm, even if it only lasts one shot is a silencer.
any device capable of reducing a firearm discharge, by even one decibel for one shot, is subject to the strict regulations of the silencer laws, regardless of to what it is, or is not, attached.
1db is next to nothing, simply putting a longer barrel on a firearm reduces the report by more than that. That means anything they say is a "silencer" is a silencer, because absolutely anything attached to a firearm, even a longer barrel or empty tube would reduce it by at least 1 db.
However the definition of them in law is one used for a firearm to reduce the report. An airgun is not a firearm. Ironicly that also means the ATF has no jurisdiction over them, meaning they also can't rule they are acceptable either. They only have jurisdiction if it is a firearm or firearm supressor, at which point it is illegal anyways (without NFA process.)
People say they are legal, because they are in fact legal for use on airguns. However if the ATF declares one illegal in your possession it at that moment becomes a firearm supressor and is illegal for all intent and purposes. It will reduce the muzzle report on a firearm by at least one decibel for at least one shot, even an empty cylinder would do that.
Much of the airgun community consensus is that integral suppressors built into the design are less grey area than removable or attached accessory ones.
I will repeat EVEN A LONGER BARREL (Chart below to demonstrate, the report on most firearms change by a few decibels just by adding or subtracting a couple inches of barrel) OR EMPTY CYLINDER WOULD REDUCE THE REPORT OF A FIREARM BY 1 decibel, so of course something actualy designed to deflect and reduce sound will do that.
In fact that is how the "quiet gun" is quiet without a suppressor, it just has a very long barrel. So even just a barrel extension could be ruled a "silencer" if the ATF chose, or one of those fake suppressors just for looks that are hollow tubes. So you better believe an actual suppressor (for an airgun) even if made out of tissue paper that would burst into flames the first time you used it can be ruled a supressor. It would reduce the report of the firearm by at least one decibel for at least one shot.
The quiet gun (shotgun), has a dramaticly reduced report just through use of a very long barrel.
Here is a chart I grabbed from
http://www.freehearingtest.com/hia_gunfirenoise.shtml
Table 1. SHOTGUN NOISE DATA (DECIBEL AVERAGES)
.410 Bore 28" barrel 150dB
26" barrel 150.25dB
18 _" barrel 156.30dB
20 Gauge 28" barrel 152.50dB
22" barrel 154.75dB
12 Gauge 28" barrel 151.50dB
26" barrel 156.10dB
18 _" barrel 161.50dB
Notice a 28" barrel has 151.50 DB firing what is 156.10 decibels out of a barrel of 26" only
2" shorter. A difference of 4.6 decibels from 2 inches of length. So a hollow tube alone would qualify as a suppressor per ATF guidelines if it screwed on the end of the barrel and they chose to prosecute.
The difference in the decibel level can also be changed by adjusting the ammo fired if a conviction is desired (remember these are some of the same people that use custom tailored ultra soft primered ammuniton to convict on possession of a machinegun by getting it to discharge twice at least one time.)
Some ammunition will have a much greater decibel difference by even a small amount of length change than other ammunition. Custom tailored ammo could really highlight the difference.
That means you could convict someone of an illegal supressor for almost anything of any size that attached to the front of the firearm (or could be "readily adapted" to do so) and did not vent the gases out of the side like a muzzle break (which although it seems louder to the shooter because more gas exits the sides rather than the front, is close to the same decibel level as the firearm without the break, negating the usual slightly reduced report from additional barrel length.)
