This is the one I was thinking of on the other post. I know for a fact the BATF would be all over anyone caught with that gadget and a pistol combined, and they (the owner) didn't have the proper FFL. Some one tested it out at a SHOT show just to see how it fit and you would not believe how fast the makers and Glock had them separated (the pistol and attachment) and the person invited to leave the Show.
Hopefully things have changed, regarding that and a pistol combined, but for now I think it's primarily sold to LEO's. The stock has nothing to do with the issue, it's the Barrel length and that has to be 16" min I think?
OK up date: (and or slight correction depending on how you want to take this): Has it happens you don't need and FFL but you DO need the Tax Stamp and that HAS to stay with the weapon or be able to be presented upon request (basically it comes down to this; LEO or BATF "May we see the Tax stamp paperwork please? Sorry but no, you can't go home to get it")