Quick google search of our state laws say the following;
In Washington you can own the following items that are regulated by the National Firearms Act
Silencers
Any Other Weapon (AOW)
Destructive Devices (DD) except explosive devices
Short Barreled Rifles (SBR)
In Washington you cannot own the following NFA restricted items.
Machine Guns*
Short Barreled Shotguns (SBS)*
NOTE:* Not legal to own or possess parts that can make these firearms unless these items were legally acquired prior to July 1, 1994 and be in compliance with federal law or machine Gun, Short Barreled Shotgun, or Short Barreled Rifle but be possessed by by peace officer for official duty, armed forces, or person in compliance with NFA who has undergone Fingerprint and background check who in engaged in the production , manufacture, repair, or testing of Machine guns, SBR, or SBS (IE: class III dealer or manufacturer)
Follow this link to find out more about Washington and NFA restrictions on Class 3 Firearms
Also for other class 3 items a state license is required in addition to the requirements under the NFA