IF your gun is semi-automatic and uses a detachable magazine...Then, it can have 1 of the following:
1) Pistol Grip. A Thumbhole is considered a Pistol Grip.
2) Folding/Collapsing Stock
3) Flash Suppressor. A Silencer counts as a Flash Suppressor also.
4) Threaded Barrel. The law says threaded for a FS, but the ATF believes that you can have a custom FS Made, so all threadings are illegal.
5) Grenade Launcher. A Flash Suppressor is also considered a Grenade Launcher and counts as 2 Parts.
6) Bayonet Lug.
BTW, removing the Pistol Grip is not as easy as it looks. The Pistol Grip holds in the Selector Detent and Spring. If it is removed, the Selector will not have any stops. Basically, it will freely move whenever you use the gun and may well come out. If you do not care about having a safety, then you would also want to remove the Selector so that it doesn't turn to the safe position or fall out while firing.