Should work. The only possibility is the outer diameter of barrel extension being too large. If I recall MM is 1.190" and a standard extension is 1.200". If you have an issue, you can probably thermofit or have the extension cut down a smidge.
As for the ridiculous WA state law that will probably pass, I would be surprised if you cannot buy a barrel. If you already have the lower, you have the gun, and, from what I have read, the proposed legislation bans assault weapon parts, but that is pretty ambiguous.