Sorry, you can' t hunt in Washington with " riffles" . Other than smacking hollow plastic balls they just don' t have the energy to down any game.....

Just teasing. Like you said as long as the rifle is at least .24 caliber and I believe has to produce 1000 ft/lbs energy at 100 yards then you should be ok. A quick call or email to the WDFW should be able to tell you for sure. Just make sure if you' re on the border you get the name of the person that gives you the info so you can quote them later in case you' re talking to an agent.