I could be wrong, but I think because it is over 50 ounces, isn't on the roster, and the foregrip is technically a "barrel shroud", it can't be a pistol here. But I'm no expert on that matter, so maybe others can chime in.
I was thinking about this the other day. If one was able to find a very cooperative FFL, maybe (again, I'm no expert) you could have them pin a faux suppressor on the end to meet OAL for the barrel and put a A2 stock on it. Then I think it's a rifle. You buy it, Form 1 to SBR and remove the pinned faux suppressor, and viola, a real HK MP5 in its natural state. There are preban mags out there for this gun, and genuine HK ones are date stamped which is a nice piece of mind.