No need to make it complicated. Guns are property and in WA State no community property agreements are necessary. If you are bringing property or assets into a marriage and want to keep it separate then you enter into a Pre-Nuptial Agreement. Even at that, given enough time in the marriage many of these "assets" are considered co-mingled and part of the Community.
If your spouse is a prohibited person then you have a problem. First of all you had to make sure they never had control of he weapons by securing them in a safe that they had no access to. Upon your death the weapons will have to be transferred to another heir that isn't prohibited from possessing them or disposed of.
IANAL but have a little experience with Community Property, Divorce, and recently had a similar discussion with my Attorney when getting my will prepared.
Washington State is pretty simple when it comes to Community Property. Like my wife says, "Half of what's your's is mine and so is the other half"