Gamo makes a air gun available in the US that comes with a "suppressor" built onto the end of the barrel. It is the Gamo Whisper. From what I can see, the "suppressor" is built onto the barrel and can not be taken off and then put onto a rifle. I'd think that this would be the big hangup on the ones the BATF tested. If it works on a paintball gun and can also be used on a real gun, too, it is a suppressor. If it works on a pellet rifle and there is no way to take it off and use it on a real gun, apparently, they have deemed that acceptable on the Gamo model. However, http://www.beemans.net/silencers_on_airguns.htm
would seem to disagree with my logic and has BATF court cases to back it up.