In Washington you are actually allowed to use sabots.
As in the past sabots are allowed. Any type of projectile is allowed.
The rest of what you say is accurate though. The other regulation is that the ignition source has to be exposed to the weather. Personally though I would prefer it if they did not allow sabots and did not allow in-lines either. Primitive weapon should be just that, a side lock, not a modern in-line that has been modified just enough to side step the laws. You can still use a modern in-line during "modern rifle" season, so it's not like you can't use it at all. You just can't use it during primitive weapon season.