Here is the 4-1-1. I own a few of them suppressor tube thingys you speak of.
#1 is about the closest to being correct.
The law (RCW 9.41.250) states that it is illegal to use any device or contrivance to muffle the report of a firearm.
So, yes, it is legal to own (with proper paperwork ,etc). It is legal to attach to the firearm. It is illegal to USE the device inside of Washington. You may take it to Oregon or Idaho and use it there until your heart's content.
Footnotes:
1. There is no LEO exemption. Every time a LEO uses a "can" in WA they are breaking the law (as my friend says "It's great to live in a police state so long as you're the police.")
2. It is a Class C misdemeanor if you use it. You will likely lose the can and the gun it was attached to if you are caught.
3. There is no exemption for manufacturers to test their own, legally made products. Nor for dealers demo-ing their wares to LEO's. But they do it anyway.
4. I know people at many public ranges that use them openly (including at the Seattle PD range). Most LEO's that know anything know it is a stupid law and so long as you possess it in accordance with Federal Law... they don't care. But.. don't construe that as my personal authorization for you to go shoot your can. Do it at your own risk.
In closing. Suppressors are a lot of fun. Other than a .22LR they ARE NOT Hollywood movie quiet except with subsonic ammo which gives them all the knockdown power of a .22LR anyway so... Owning suppressors is less about being a stealthy, sniper like figure and more about exercising your rights as a gun owner to fullest and also experiencing all the joys of shooting without the annoying muzzle blast and cumbersome hearing protection (keep the eye protection though... cans blow a lot of crap backward out of the firearms action). The best part is actually hearing the bullet travel downrange and hearing the sound of it impacting its target. It adds a whole new dimension to shooting.
Expect to spend about $1,000 on your first suppressor purchase (to include purchase, tax, transfer fees, threading of barrels, etc.) maybe $700-$800 if you buy a .22LR can first.
I fully recommend that you make your first suppressor purchase a .22LR. It will be the most versatile, and fun can you own. Learn from me... I made it my latest can purchase... wished I had done it sooner instead of large caliber pistol and rifle cans.
Finally, who do I recommend for cans?
www.gem-tech.comThey make the best and the Outback is one of the best .22LR cans you can buy for the money. Remember... you have to drop $200 bucks on this thing in tax alone. Don't settle for some $150 knock off because when you realize how much you hate it and want to get a Gemtech.... you'll have to drop another $200 in tax.