I thought of this ten years ago. I just don't have the machinery to build anything yet, perhaps some day... Lots of ideas in my head I'd like to turn into metal. It was a way to attach a Glock to the rail system just before doing entry. Idea was to use Swan mounts so it could pop on and off real quick --you wouldn't want to walk around with it. Entry tool only. It also sounds like a good Red Jacket project (maybe they could put three on there and make 'em spin around the rail system!) but it does have a realistic, albeit limited, legitimate use.
As for the legality, I would have thought the same thing. But there is actually a letter posted on some forum (perhaps here) where a guy asked the ATF if mounting a Masterkey under an M4 makes the Masterkey an SBS. They said no and he posted the letter. Basically they said that each weapon has a serial number and that is what defines what the item is. So they way they interpreted is this: if you put a stock on a Masterkey, it becomes an SBS. If you put a Masterkey on an M4, it is an AOW mated to an M4.
If memory serves me correctly, there was a rifle/shotgun combo that used to exist that was banned, an Air Force survival rifle maybe? I THINK it had a 16" bbl., but since the shotgun barrel had to be 18", it was illegal. The two barrels shared the same serial number. But I suppose had they built the rifle, then sold an AOW that attached to it, then it'd be okay.
Still, based on the way those crooks work, interpreting the laws however they want, making up whatever they want, etc., I would most DEFINITELY find that letter, copy it, and send it to the ATF requesting a letter for yourself. Even then, as I understand it, you can have the letter from the Technology Branch saying it is okay, then go to a range that has an ATF guy there and STILL get arrested for an illegal NFA item.
AND, they've also been known to approve a product, only to reneg on it later. Like those bump fire triggers; they sent a cease and desist letter to the owner, raided his business, got all of his customer's info, and then sent them threatening letters telling them they had to turn them in. This happened most recently to a guy that was selling handloaded ammo advertised as AP. Other people sell the same ammo but don't call it AP. They took his customer list and sent threatening letters to everyone saying any unfired rounds had to be turned into ATF. But if you loaded your own or bought them elsewhere, where they weren't advertised as AP, then you are okay. Get it? I didn't think so.
Damn if you, damn if you don't.
If I wanted one, if I needed one, I'd get the letter. I really could have used that in the army (my design anyway, I haven't seen the one you are talking about) but have absolutely no use for one now. Way too specialized of gear for me, but a great idea for soldiers. Too bad it came out too late.
