Lemme answer this way-
around here we usually refer to OC as "of a loaded weapon"
You may NOT carry a loaded long arm in or on your vehicle in WA
Now, if I interpret your post a bit, you may be asking if you can transport your long arm via M/C. You can, and I do. As I said above, it must be unloaded. Mine is cased when carried on a M/C. Mainly, then there's no telling what might be inside. It might be my muzzlestuffer, or it might be my Mini-14. No one can tell. Ammo goes in my saddlebags.
I s'pose you could OC a long gun walking, but it'd be a mite inconvenient. I believe that's why pistols were invented.