Things
Forfeiture of Firearms.
There are many ways in which you can be ordered by a Court to forfeit your firearms. This can be done even if you have not been convicted of a crime that for would forfeit your Second Amendment Rights, or before you are convicted of any crime at all.
For more information about Washington's firearms forfeiture law, https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.098
Frames and Receivers.
Looking to buy a frame or receiver in Washington State? Looking to by an 80% lower in Washington State? https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.111
Obtaining an Unfinished Frame/Receiver in Washington State
The sale, offer for sale and importation of 80% lowers is illegal in Washington state. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.327
Adding a Serial Number to an Unfinished Frame/Receiver
Yes, Washington law does permit you to take an unfinished receiver to an FFL, who would then serialize it, and return to you, after of course you clear a background check and undergo the waiting period. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.328
Serial Numbers / Identifying Marks.
You cannot alter or obliterate any serial number on any firearm. Period.
https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.140
Coyote Getters. Yes, there is a law for that.
https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.185
Other Dangerous Weapons
https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.250
Exceptions for law enforcement, firefighters and military.
https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.251
Unlawful Firearms.
Washington state has, unfortunately, banned many platforms of firearms or in other instances significantly restricted them.
Possession of any of the following weapons is illegal under Washington law. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.190 And in some instances, merely having parts which are designed to be components of a banned firearm in Washington state also constitutes criminal conduct. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.220
The Use of Banned Items in the Commission of a Crime.
The possession of automatic weapons is illegal under Washington law except is some very limited circumstances. Using a banned item in the commission of a crime is a very serious violation in Washington state. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.225
Untraceable and Undetectable Firearms.
The difference between an untraceable and an undetectable firearm is that an undetectable firearm is based upon what it is made of, while an untraceable firearm deals with whether that firearm contains a serial number or not. You can see the actual definitions here. https://app.leg.wa.gov/rcw/default.aspx?cite=9.41.010
The possession and manufacture of these items is illegal in almost all circumstances. https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.325 Additional restrictions include https://app.leg.wa.gov/RCW/default.aspx?cite=9.41.326