According to Washington State Law, it is illegal to carry a loaded rifle or shotgun in a vehicle unless:

(5) For purposes of this section, a firearm shall not be considered loaded if the detachable clip or magazine is not inserted in or attached to the firearm.

http://apps.leg.wa.gov/RCW/default.aspx?cite=77.15.460

So my question: Does this mean that you can't have rounds in say an elastic buttstock pouch? Similar problems with shotgun rounds in a buttstock (speedfeed) or a magazine in a buttstock ammo pouch?

Reading the statutory language. it seems clear that the buttstock carriers etc are NOT considered "loaded" so long as the shotgun's mag tube is empty.

The statutory language however "MIGHT" indicate that an empty AR-15 with a mag on those buttstock pouches be considered loaded.

Go to the library legal section and look up the reporter using the statute number.
There are law books that define the duties of the state to make a case.
You can also look up in the court reporter, but many lower cases are not in courts of record.

detachable clip or magazine

Your buttstock is not a magazine, if it is you got some weird ass gun.  You should have no problem transporting it, why not add a side saddle to your shotgun and throw some rounds in that too.

Only clips, magazines, or tubes count for being a loaded.

Quoted: detachable clip or magazine Your buttstock is not a magazine, if it is you got some weird ass gun.  You should have no problem transporting it, why not add a side saddle to your shotgun and throw some rounds in that too. Only clips, magazines, or tubes count for being a loaded.

Well here's the part that's confusing me

attached to the firearm.

What does attached mean?

Also, does a backpack count as an opaque container for a handgun? Also, can I say have a handgun in a small case, and have a loaded magazine right next to the gun?

I think the nature of the laws are to require more than pulling a loaded handgun and shooting it at deputy. I think in short, they want more than 2 actions in order to have a loaded gun in action? You have to remove it from case, then load it?