WA state has no qualifications to own a silencer, they are not even defined as guns in WA. They just ban their use as a gross misdemeanor. At the federal level, anyone who can own a gun can own a silencer. The applications are routinely approved by the ATF for anyone who fills out the forms correctly. You need to be at least 18 to make or own one, 21 to receive one from a dealer.
I make silencers on ATF form 1's. I live near Seattle, far from any borders. Any time I leave WA for one of the 36 states where they are legal for unlicensed civilians to use, I bring as many guns, ammo and silencers as I can carry with me in the car or plane. Northwest Airlines limits you to one machine gun, one silencer and two rifles and pistols according to their website. They have not enforced this when I exceeded the number allowed.
United has no limit, just get them into a locked hard case.
The law banning use is rather lame. I have heard the police and military are exempt from prosecution for silencer use, but have not been able to find any loophole in the law that allows this. I wrote to the WA AG requesting that he extend the same courtesy to those who own registered silencers and was told there is no exemption from prosecution for use by the police or military. Anyone know more about this?