That's really splitting hairs - as you're relying on the fact that the firearm in question would not be considered a rifle because, having no buttstock, it is not designed to be fired from the shoulder.
While you're most likely technically correct - I would strongly recommend against such a configuration here in NJ. It's along the same lines of the California-style "bullet button". The argument there is that the bullet button makes the magazine non-detachable, and therefore nullifies all other "evil feature" requirements. No one's ever tried to implement one here in NJ, and my argument against them has always been, "do you think a jury, after watching a prosecution witness demonstrate a magazine dropping free of the receiver after actuating the bullet button with the tip of his pen, would consider said magazine 'detachable'? If so, you're going to prison.".
The same applies here... the prosecutor holds up the scary-looking rifle, minus a buttstock. In the eyes of every single member of that jury (that's made it through voir dire , that is), it's an assault rifle, and you're guilty.
I know the above can be applied to just about any aspect of firearms ownership here in NJ... I know. It's just that, to me, that's far too fine a line to try and hang your freedom on - even if, by the strict letter of the law, it may not even be illegal.