So DSI on Long Island has submitted fixed mag AR pistols to the Suffolk and NY State PD for their "approval". It uses their billet lower that requires the trigger group be removed and a set screw be removed (with a long a** allen key) to change out the mags. We all know what the ATF thinks, but in NY, anything can be anything on a whim, so it's probably worth waiting to see what the official response is to DSI's test cases. I can't find anything in the law that says an AR pistol without the brace but with just a pistol buffer tube would be illegal if it's on a fixed mag lower.
Making a "firearm" (by the ATF's definition) where you have a fixed mag AR pistol that's longer than 26" because of a 10" or 11.5" barrel and a vertical foregrip could be an issue though. According to penal law 400, it's considered a firearm because of the < 16" barrel, and all firearms are illegal in NY. We have our regular pistols because of the pistol/revolver exemption at the bottom of that section via our our licenses, but because the > 26" gun is not a pistol/revolver, but a firearm, IMHO, we're not exempted and it wouldn't be ok.
http://www.dark-storm.com/firearms/ds-15-pistols/