I think Casca (who has practiced law) said that 'Yes, airsoft silencers are legal, they are not Class 3 devices.' in a previous thread about this. It's come up a few times recently.
Yes, my SOCOM and my KA lightweight and my Delta silencer all have caps on the ends that screw off. All of them will thread directly to the end of an AK (real steel). None of them would hold up to the gas pressure of a real firearm.
Like Scarlet Army said, if you can buy it off
www.airsoftgi.com with no hassle, then it isn't illegal. If it were, importers would have a big problem getting them in, and retailers would have a big problem selling them.
Don't worry about it.
P.S. If someone wanted to get me a full book bound copy of the OR Penal Code and Criminal Code of Procedure et. al. then I wouldn't mind reading through it for stuff like this. (I have had some experience reading 'legalese').