18 USC 922(r) is a strange one. If you want to be totally safe, follow the parts count. I'm not aware of any individual gun owners being convicted for violation of 922r. 922r simply states that it is unlawful to manufacture a firearm which would be prohibited from importation under 925d. 925d prohibits importation of foreign made firearms which are not sporting purpose. Two questions arise: 1) what is sporting purpose, 2) how many foreign parts can a firearm have before it is foreign made? Neither of these questions are answered by the laws passed by Congress. The Secretary of Treasury wrote regulations to answer these questions: 1)a sporting purpose rifle does not have a separate pistol grip, bayonet mount, etc. 2) for a rifle to be considered domestic manufactured, it must not have over a certain number of foreign made parts, hence the parts count list. Any person or business operating under the authority of a Federal Firearms License (importers, manufactures, dealers, gunsmiths) must follow these regulations. Does it apply to what individuals do with their own firearms? If you asked the ATF, they would say yes, but such is the nature of government agencies to extend their authority as far as they can get away with. Ultimately this would be for the courts to decide. I think the same logic would apply as the Supreme Court used in US v Lopez.
I'm not a lawyer. This is just my personal opinion. You are responsible for your actions, just as I am responsible for mine. The NFA is a totally different law, I would not apply the same argument.