It matters not how the device works or whether it is an "attachment" or not. The only thing the law concerns itself with is that the device's intended use is to"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 foruse in such assembly or fabrication."
.diminish the report of a portable firearm