Re: WHY paintball silencers are illegal...an article
Actually, let me clarify that. The STFU barrel is legal as it is not a barrel in normal senses. It is sealed on the front end. If you were to modify the barrel to actually fire, it would be HIGHLY illegal, but otherwise it's in the gray area towards legal. It's for dry-firing only, and no projectiles (balls or otherwise) are fired, so it is not a "silencer" by definition.
Keep in mind, though, that at the moment that it becomes an issue and an officer is to make a field decision on that, it is up to HIS discretion and he might call it a "silencer" and you could be missing it for a while until the courts decide that it isn't. Just a fair warning.