according to ATF regulations, if it even lowers the sound by one (1) decibel, then it is considered a suppressor and requires a tax stamp, fee and registration. doesn't matter whether the suppressor is intregal or an add-on type, if it reduces the sound it's a suppressor.
if you stick a 2 liter plastic soda bottle over the end of the barrel of a firearm and it reduces the sound, it's suppressor. anything that can reduce the sound, is a suppressor. if it's not registered with the ATF, then it's illegal.