The one thing being left out here, is that 922r is an IMPORT regulation. Once it's imported, it ceases to have jurisdiction. If the feds tried to prosecute someone, they would hope you didn't know this and they would probably get away with convicting someone who didn't realize they were being treated as if they were someone who brought the gun into the country and "assembled" a "non sporting rifle" (terms in quotes have specific legal meaning within this particular section of code). THAT'S what 922r covers: the assembly of "non sporting rifles" with respect to importation. Most people either don't know, or forget that importers are actually allowed to import "non sporting rifles" temporarily so they can change them to "sporting configuration." EVEN though those rifles have been imported (using the ordinary sense of the word not the legal sense), customs gives them a temporary legal designation of not ACTUALLY being "imported" yet. Thus, we see there is a clear legal change of jurisdiction going on here. When they haven't been "imported" yet (even though they're sitting in an importer's warehouse in some state, we'll say Florida), those rifles are actually under federal jurisdiction. Once they are retrofitted by the importer into "sporting" configuration, and pass to normal inventory, they are now "imported" and are past federal jurisdiction. Past this point, 922r has no jurisdiction. All these people stressing about "parts count, parts count, parts count" are complying with a section of code as if they were the importer when they are not. Once something has been imported, import regulations such as 922r don't recognize it's existence. Once something has been imported, import regulations view the item imported, and your neighbor's front lawn the SAME, i.e. it doesn't view them at all.
The feds take advantage all the time of people's ignorant and false legal assumption that "once in federal jurisdiction, ALWAYS in federal jurisdiction." If you plead guilty or plea bargain a 922r issue as a non importer, you're essentially declaring yourself to be an importer as it pertains to THAT charge, and declaring that took imported parts, assembled a "non sporting" firearm, then imported it. The feds as well as local prosecutors absolutely LOVE it when people make such declarations that put them in the code's and the prosecutor's jurisdiction. It makes their job easy and pads their conviction/plea count.