How does that work, legally speaking? Don't you have to NFA the weapon somehow before you can put a stock on it?
Nope, as long as the bbl is over 16" and the overall length is over 26" it is perfectly legal to convert a pistol into a rifle. The problem is ATF recently sent out a letter that kinda fouls up a lot of things.
Way back in the day Thompson Center came up with the idea of selling a pistol and carbine kit, basically one of their Contender pistols with a short pistol bbl, a longer 16"+ rifle bbl and a buttstock that replaced the handgun grip. The idea was one could swap parts around and have 2 guns with only one frame. ATF said, even though the kit instructions specificaly advised the owner never to install the buttstock and short bbl together, that because one could POSSIBLY install the short bbl and stock together, it was in fact an SBR (short bbl rifle) and had to be NFA registered as such even if the owner never assembled it that way. TC registered one kit, then sued ATF if federal court for a tax refund based on the fact that it had never been assembled as an SBR and therefore wasn't one. Went to the Supreme Court and they ruled for TC, said as long as it was never assembled as an SBR, it wasn't one. Obviously, the assembly as a rifle and a handgun could be done at will, and switching back and forth from rifle to handgun was allowed because the receiver started out as a handgun receiver.
Normally, if a receiver is first assembled as a rifle, removing the buttstock and shortening the bbl does not make it a handgun, it becomes an SBR. The TC case changed this, creating a special exception for handguns that are converted into rifles, if you reconfigure them as a handgun they become a handgun again, not an SBR. Since the TC case, the manufacturers have sold numerous handgun/carbine kits such as the Mechtech and Beretta.
Silly them, they asssumed that a SCOTUS precedent applied to all. ATF recently sent out a tech branch opinion letter stating that the SCOTUS case did not set a precedent and only applied to that particular TC kit!
That's like saying the recent Heller case only allows Dick Heller to keep a gun at home and the rest of the DC populace have no second amendment rights!
So, as it stands right now, ATF considers inserting your Glock pistol lower onto a Mechtech rifle upper the making of a rifle, if you later reinstall the handgun slide and bbl, you have now made an illegal SBR! They would likely lose in court if they pressed it, but no one wants to be the test case.