There is only one precedent from the lab on this one - the Taurus revolver carbine. The lab ruled it restricted, despite it being over 26.5 inches over all and having a shoulder stock. There's no real logic involved, the answer you get reflects nothing more than the opinion of the minor Deity who looked at the gun on the particular day. The lab opinion also isn't permanent, if they rule it Non-res today, they could change their hive mind tomorrow and decide to prohibit and confiscate, or simply restrict it. In a similar vein, if anyone can tell me how those dumb 'Mares Leg' cut down rifles are non-res while the Taurus carbine is restricted I'll buy them a beer.