In addition to that police officer, my neighbor is a deputy Sheriff and he looked into it for us. It depends on the intent of the user. Unless you are firing at a living thing or someone else's property, they are considered a piece of recreational equipment, in the same sort of class as a model rocket. I think that cop was just trying to scare you with the pipe bomb thing, because we looked into it pretty heavily and never saw that precedent.

This is what the Feds have to say about them...

"We have previously examined that certain muzzle loading devices known as "potato guns." These potato guns are constructed from PVC plastic tubing. They use hair spray or a similar aerosol substance for a propellant, and have some type of spark ignitor. We have determined that these devices, as described, are not firearms provided that they are used solely for launching potatoes for recreational purposes. However, any such devices which are used as weapons or used to launch other forms of projectiles may be firearms and destructive devices as defined. "

And I can find no statute in Washington state law that bears one way or another on the issue, so my understanding was that outside of any city limits, don't shoot people, animals or property, and you are fine.