This is simply not true.
All you have to do is pay a $200 tax and fill out a form for SBR.
ATF regulations CLEARLY allow certain Lugers, Mausers and Browning Hi Powers to have an attached stock WITHOUT the owner having a tax stamp.
If you pay a $200 tax for the SBR it's no longer a pistol and my comment is still accurate.
The OP and others reading this thread are not interested in attaching a stock to a very specific model relic gun. They want to attach stocks to their Glock and other modern automatics. So I felt it was important to clearly state that you simply can't do it rather than continue to bog down the thread with NFA regulations and perhaps give some kid the idea that maybe he can attach the stock to his Glock.