I think that the problem with some to the definitions above is that many things could be theoretically be used as a suppressor. For instance have you ever heard of a soda can suppressor? Well last I checked you don't need to send forms into the ATF to buy a coke.
Also can't some baffles in basic suppressors be made from washers available in a hardware store? If so shouldn't you need a form for these? I'm not trying to say that what the people above posted is wrong because I agree with them but I think it goes to show how screwed up out national firearm laws are. Also on a side note I live in NJ where BB guns, pellet rifles, and even slingshots are regulated as actual firearms and require an FFL:barf:
!!!! I wanted a Gamo Whisper but in my state that is a Class 3 weapon