REALLY, guys.. for the 3rd time this week.. if you don't understand the rules don't post opinions in this forum. It's mind boggling to see guys with 1500+ posts make blatantly wrong statements.
Now onto the question and the RIGHT answer..
P99AS9, you want to complete a "Form 1" to manufacture a "Short Barrel Rifle". You can do that with the GSG-5 pistol. It's very easy. You can get the form from titleii.com and instructions for filling it out are all over the internet and this forum. It's essentially the same as buying a silencer. AFTER you get the approved form back and your $200 stamp you can buy and attach the butt stock. DO NOT BUY the stock before hand. That could be seen as constructive possession.
This is what your GSG-5 SBR could look like: