The magic rule is 10 foreign parts or less. A factory S12 has 14 foreign parts but is exempt from 922r because it's considered a 'sporting firearm'. You go slapping a 10rd mag in it and they no longer consider it a 'sporting firearm'. Same goes for adding a PG; it's no longer a SF.
So you decide you want to convert one into a 'non-sporting firearm'; you'll need 5 U.S made parts to be legal.
-A pistol grip does NOT count towards your count
-A Buttstock is compliant
-Trigger is compliant (you'll need one for the conversion)
-A 10rd. mag = 3 parts
So long as you run 10rd. mags in it, you'd be perfectly legal. Swap in a factory foreign 5rd. mag, and you'd need 3 more parts to get legit.
-a gas puck is an easy and cheap one
-I can't remember if the Tapco G2 trigger group comes with the hammer and disconnector, but if it does, those count as an additional 2 parts.
-An aftermarket forearm is another.
Anyways, as you can see, it's extremely easy to be compliant when converting one. If you have any more questions, let me know.