My experience has been with paintball guns and is about 10 years old now, maybe things have changed since then. At that time, suppressors and silencers were regulated by the ATF as a type of destructive device. Regulation was based on design (if the device used a design classified as a suppressor, it was a suppressor), and on function (if the device reduced the report by some arbitrary amount, it was a suppressor). The paintball dealer I knew wouldn't carry silencers for .68 caliber airguns unless the ATF provided a letter saying they didn't consider the silencer a silencer. He sent several examples to the ATF with an explaination of the circumstances (paintball gun, really big caliber, etc.). In every case, the ATF decided the silencers were silencers, and therefore basically illegal. I remember one manufacturer was arrested for building paintball gun silencers, but he was also using the Maxim design, which may have been a factor.
I never saw a paintball suppressor that actually worked that wasn't classifed as a silencer by the ATF.