From SAF.org:

“We took this action due to the confusing and arbitrary language and nature of I-594,” Gottlieb explained. “Three of our plaintiffs, including my son, are residents of other states and cannot legally borrow handguns for personal protection while traveling in Washington. Under I-594, all transfers must be done through federally-licensed firearms dealers, but under federal law, dealers cannot legally transfer handguns to residents of other states. I-594 also essentially prohibits our non-resident plaintiffs from storing their own firearms here.