1. YES! They are legal
2. Correct! You can own it but you can't use it. Define use: Well the law states that it is unlawful to use any device or contrivance to muffle the report of a firearm. So can you attach it to the gun and not fire it.... so far as I can tell... yes you can.
3. There are owners, sellers, and even LEO's I know that will tell you that as long as it is properly papers they don't care and they won't tell if you shoot it in WA. Do so at your own risk. You won't get slapped with a felony but you will more than likely lose the can if caught.
Otherwise. Come on in and join the party. Suppressors are a lot of fun. And totally legal to use in many other nearby states (Oregon, Idaho, Montana (I think), Alaska). I tell people that your first can in any caliber other than .22LR will cost you about $1,000.
I purchased and SOS-45 from
Gemtech for my first. The can was $595.00. $200 for tax. $150 for a threaded barrel and $50 to form a corp because I couldn't get a CLEO signoff. Personally I think the corp is the way to go. No figerprints and no photo hassle on the Form 4. For others it may not be the way to go. Investigate all the pros and cons and then decide for yourself.
In hindsight I really wished I had bought a .22LR can first. They are generally cheaper and can be put on a host or relatively inexpensive firearms. If you are looking for that "Hollywood" level of quiet then a .22LR with subsonic ammo is really the only way to do it. Anything larger will still have a noticeable level of sound to it.
There really is so much to go into on the subject that I don't have the time to type. But suppressors certainly are fun. I think everyone should have one.