Is it legal to store a firearm out of state with someone you know to be of good character (i.e. not a pprohibited person)?

I don't see why not. What reasons would you have to think that it's prohibited?

When you say "out of state"

What state?

Overall, no but then you fall under their state and local laws for firearm ownership (having a firearm).
For example parts of NY you couldn't even have a magazine... let alone a firearm.

If it is an NFA item, and they are an NFA holder, and they are listed on Your trust...Then yes....If it is just a standard firearm, and it is not a prohibited item in that State, Then yes...I would have a notarized statement of ownership listed with both parties, and the firearm(s) listed on some insurance somewhere, just to protect everyones interest in case of fire, theft, or it "Just Disappeared".....Have that situation in My life at the present time...

Excuse the necro post, but what about just storing it in a rented space, like a storage unit?
If i wanted a firearm that isn't legal in WA state could I purchase it in another state and simply store it there?
Not a financially viable thing for me right now but it would be interesting to know its an option.

....... legally yes..

Would I Personally.. Never..

Storage units can easily be broken into.

Federally anything NFA I highly doubt could be legally..

I personally if I had an NFA item, I would want it under my roof. So, unless the laws in WA change I might have to move out of state if I win the lottery. Cause if I win the lottery I will buy a big chunk of land, and a Ma duece

What firearms?

I'm buying a boat just to have a boating accident.

For all practical purposes it should be legal as long as you don't actually "transfer" the weapon (using gift or sale as a definition of transfer) to another person in that state. Under ATF rules you can ship a firearm to yourself in care of a person in another state as long as they don't open the package.

Some ranges provide lockers for their out of State patrons. (Front Sight in NV was doing this for a lot of their CA customers).

As I understand it, any firearm stored in another state will be subject to the laws of that state.

I prefer to keep my firearms under my own control so I wouldn't even think of storing one out of state (unless buried in a secret area of some national forest in a "doomsday vault" ).

I bet that's where the I-594 authors got the idea for writing the range exception the way it is: ". . . (f) The temporary transfer of a firearm (i) between spouses or domestic partners; (ii ) if the temporary transfer occurs, and the firearm is kept at all times, at an established shooting range authorized by the governing body of the jurisdiction in which such range is located; . . ."

I know a lot of folks read that section as meaning "kept at the range during that day's shooting," but that's NOT what I-594 says. "At all times" means "at all times" and the only practical way for that to happen is with lockers at the range.

Definitely not legal in CA, unless where stored has zero access to the firearm. Others, it is a transfer.

I think it was more for those Ranges that own a large number of firearms for various competitions. Kenmore Range for example owns a bunch of M-1 Garands, some .22 Pistols, and some .22 Target rifles. Could you imagine how long a "match" would take if each and every "competitor" had to undergo a background check prior to checking out the firearm and likewise when turning it back in to the "Armorer".