County no shooting areas are different county to county, some let you shoot if you are hunting. However, in Snohomish County you can't discharge a firearm for hunting purposes in a county no shooting area. Additionally, it is unlawful to discharge a firearm on Snohomish County park land. Also, under the Snohomish County code any county lands identified as "undeveloped parks" are closed to public access.
Just because you've seen someone do it, doesn't mean it's legal.