According to 18 U.S.C. 921(a)(24):
The terms “firearm silencer” and “firearm muffler” mean any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for use in such assembly or fabrication.
According to this ATF ruling, even if a device was not intended to “silence, muffle, or diminish the report of a portable firearm,” when it becomes readily usable for that purpose, it is a sound suppressor under the above US Code. The device in question was a paintball gun “suppressor” that reduced the sound of a .22 by under 8 decibels.
No definition of actual sound reduction required to be considered illegal was given in the ruling. Therefore, we may assume that any device capable of reducing sound by even a single decibel is illegal. There are no allowances for devices that were not ostensibly intended to reduce the report of a firearm, but do so as a consequence of their design.
Unfortunately for owners of (a certain muzzle brake) such as myself, in my recent testing, it proved to be one decibel quieter than the same weapon without a muzzle device (This is in line with other informal testing that I have seen conducted by people such as myself – however, it is more offensive to the ears of some people because of the frequency of the report, not because of the number of decibels emitted).
Were I to continue using this device, knowing that it apparently diminishes the report of a firearm, it would seem that I would be committing a felony.
I must make it clear that I will continue to use this device and that this post was created solely to point out the ridiculousness of what is apparently interpreted to constitute a “silencer” under 18 U.S.C. 921(a)(24). See this page for information on a related topic, although the following would seem to indicate that since the brake does not have any of the following features, it would generally not be considered a silencer on its own:
General characteristics of known firearm silencers include:
1 Ported inner tube (bleed holes)
2 Expansion chambers
3 Baffles or washers which create seperate expansion chambers
4 sound dampening material such as foam, steel wool and other materials
5 End Caps