According to Freedom Fighter Tactical, these are the parts listed by the BATF for achieving 922(r) compliance when you decide to "mod" the gun to increase shell capacity. To learn more, go their website and select "Learn" or "Videos" from the top menu. Videos are long but informative.
Receiver
Barrel
Bolt
Bolt carrier
Gas piston
Trigger housing*^
Trigger*^
Hammer*^
Disconnector*^
Buttstock (with integral pistol grip)*^
Forearm/handguard (forend)*^
Magazine body*^
Follower*^