Washington state does not have a requirement to register firearms. In short, you don't register guns there. Would suggest you visit a local gun shop for information on Washington State and firearms.
No, it is not illegal to not get a pay stub in Washington state. Washington is one of the few states that does not have pay stub state requirements.
Washington State does not require registration of ordinary Title I firearms.
No, polygamy is illegal in Washington state.
Potato guns leagal in Washington state
yes
Laws vary from state to state. In MOST states, there is no "registration" of firearms as such. If you live in a state that DOES require registration, then yes, that would be illegal.
All firearms are legal when they are manufactured. Illegal comes into play when you start looking at federal, state and local laws.
Either the Federal or the State Government can make owning certain firearms illegal with either Executive Order or Congressional Law, but the Federal laws will supercede any and all State laws unless the Judicial Branch rules it unconstitutional.
yes
They're not.
yes