That is BAD information there. ANY shotgun with barrel(s) under 18" in length IS an NFA firearm.
I believe my statement is correct regarding WA law. The Serbu Super Shorty is a "factory short shotgun type weapon", but since it was never made from a shotgun it is legal in WA. It's not a shotgun per se, it's an AOW.
I shouldn't have said "they aren't covered under NFA" however, but my point was that you can buy them over the counter in WA, unlike other NFA weapons such as SBR and SBS.
The WA Attorney General already said these weapons are not SBS under WA law back in 2007.
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In summary, the weapons described in the attachments to your request are not "short-barreled shotguns" under state law, because they were never intended to be shoulder-fired and therefore were never "shotguns" as defined in state law. An originally-manufactured firearm with the hybrid characteristics of a pistol, rifle, and/or shotgun does not meet the statutory definition of a shotgun if, when manufactured, it did not possess all the characteristics of a shotgun and therefore would not be properly classified as a "short-barreled shotgun."