One last thought but you have to research this thoroughly! You can build one for youself (and only ONE),serialize it and register it with the BATF. I believe without the tax but I'm not sure. Has something to do with the commerce clause and the absence of commerce. There is a strict time line from the fabrication point at which it becomes legally a supressor to the time you regiter it (24 hours if I remember).

Here's an interesting thread about silencers and the ATF...

http://rimfirecentral.com/forums/showthread.php?p=551331#post551331