....This appears to regulate transfers within the state of WA. Which would make sense that WA does not have to power to prosecute actions taken outside its borders....
It's not necessarily true that Washington State can't prosecute actions outside its borders. For example, see Washington State, RCW 9A.04.030::
State criminal jurisdiction.
The following persons are liable to punishment:
(1) ...
(2) ...
(3) A person who being out of the state, counsels, causes, procures, aids, or abets another to commit a crime in this state.
(4) ...
(5) ...
(6) ...
(7) ...
It's called "long arm jurisdiction." While the principles of due process limit the extent to which a State can apply its laws to someone not in the State, there can be some meaningful contact, e. g., doing something outside the State causing an effect in the State, sufficient to justify that State applying its laws to you. So, for example, someone from out-of-state helping someone else commit a crime in that State would generally be found sufficient contact to justify jurisdiction in connection with that crime.
...This would appear to create a situation where two WA residents could make a FTF transfer without a background check, as long as the sale does not happen within the borders of WA....
That might not violate Washington law, but, if he takes the gun home with him, it probably violates federal law, specifically 18 USC 922(a)(3) which provides in pertinent part as follows (emphasis added):
(a) It shall be unlawful—
...
(3) for any person, ... to transport into or receive in the State where he resides ...any firearm purchased or otherwise obtained by such person outside that State,...
... would two residents from Spokane who met in Idaho to make a FTF sale actually be violating either WA or Federal law, since the sale is outside WA borders but between residents of the same state?....
And yes, if the buyer took the gun home with him he would be violating federal law (18 USC 922(a)(3)) by transporting into his State of residence gun he obtained outside his State of residence.
... to this would be something like a CA resident buying an AR15 from a FFL in WY to keep in his WY cabin, or temporary sporting loans outside WA borders by a WA resident. Do states have any say in the Federally legal possession of firearms when its citizens are in other states?
In those two hypotheticals the States probably have no say, but each raises questions under federal law.
In the case of a California resident buying an AR-15 from an FFL in Wyoming to keep in Wyoming, depending on all the facts, the FFL might be violation federal law to make the sale (and the California buyer might have liability for aiding and abetting the FFL's possibly unlawful act). See 18 USC 922(b)(3) which provides in pertinent part as follows (emphasis added):
(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver --
...
(3) any firearm to any person who the licensee knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the licensee's place of business is located, except that this paragraph
(A) shall not apply to the sale or delivery of any rifle or shotgun to a resident of a State other than a State in which the licensee's place of business is located if the transferee meets in person with the transferor to accomplish the transfer, and the sale, delivery, and receipt fully comply with the legal conditions of sale in both such States (and any licensed manufacturer, importer or dealer shall be presumed, for purposes of this subparagraph, in the absence of evidence to the contrary, to have had actual knowledge of the State laws and published ordinances of both States),...
So to be lawful the sale by the Wyoming FFL to the California resident must fully comply with all requirements of both California and Wyoming law without regard to whether the California resident takes the gun back to California. Among other things, California firearms transfer laws require a dealer make a particular filing with the California DOJ. A non-California FFL will not be able to comply.
But all of that might be moot. If the Wyoming cabin is a vacation home for the Californian, he might be considered a Wyoming resident for the purposes of the Gun Control Act of 1968 when he is present at his Wyoming home. For the purposes of the GCA, "State of Residence" is defined as follows (27 CFR 478.11, see example 2):
State of residence. The
State in which an individual resides. An individual resides in a
State if he or she is present in a
State with the intention of making a home in that
State. If an individual is on active duty as a member of the Armed Forces, the individual's
State of residence is the
State in which his or her permanent duty station is located, as
stated in
18 U.S.C. 921(b). The following are examples that illustrate this definition:
Example 1.
A maintains a home in State X. A travels to State Y on a hunting, fishing, business, or other type of trip. A does not become a resident of State Y by reason of such trip.
Example 2.
A maintains a home in State X and a home in State Y. A resides in State X except for weekends or the summer months of the year and in State Y for the weekends or the summer months of the year. During the time that A actually resides in State X, A is a resident of State X, and during the time that A actually resides in State Y, A is a resident of State Y.
Example 3.
A, an alien, travels to the United States on a three-week vacation to State X. A does not have a state of residence in State X because A does not have the intention of making a home in State X while on vacation. This is true regardless of the length of the vacation.
Example 4.
A, an alien, travels to the United States to work for three years in State X. A rents a home in State X, moves his personal possessions into the home, and his family resides with him in the home. A intends to reside in State X during the 3-year period of his employment. A is a resident of State X.
The loan of a firearm by a resident of one State to a resident of another is also covered under federal law (18 USC 922(a)(5)). It is however expressly permitted when temporary and for a lawful, sporting purpose.
I also need to say something about multistate situations or situations in which the laws of multiple jurisdictions could apply. There is an acknowledged branch of law dealing with such situations. It's called "conflicts of law." Whenever the laws of multiple jurisdictions are involved there will be no simple answers. The resolutions of such questions are very fact dependent and will also depend on the particular conflict of law rules of the involved jurisdictions.
A side issue could be when does federal law preempt state law. That's another complex, non-intuitive matter. I briefly discussed federal preemption principles in
this post.
