Building a MP5K in semi is not an issue with 922(r) because pistols are not affected by 922(r).
Building a post-sample is also not an issue because of the restricted access.
Building an SBR is an issue. Shorten the barrel of an existing gun is fine, building it from the ground up is not.

The problem with this law is that, by ATF's own admission, it is nearly unenforceable, difficult to prove and even more poorly understood by those in the field or not having a proper understanding of how it works. I previously stated that 922(r) has many convictions. It does. The thing about it never being the lead charge is telling. It's always an add on. When somebody is caught doing lots of other nasty things, 922(r) is part of the whole host of additional charges added on (if there is probable cause). The idea being that a guilty plea will be worked out.

How is a a local LEO supposed to know the nuances of a law that even ATF has trouble understanding? How is an IOI supposed to actually be able to know the country of origin of non stamped parts inside a weapon? Is it reasonable, prudent, efficient or even financially warranted to send in every suspect weapon to FTB for "special testing" (assuming that this is possible) to determine origin? Of course not.

The practical answer is that IOI's aren't on the lookout for 922(r) violations when they do inspections. Their training is such that they have difficulty recognizing which serial number to use on an integrally suppressed pistol, that they think a Braverman pen pistol is an AOW, that they can take the bound book off premise for review. They make mistakes, but there is no amount of training and funding available that will put a mobile metallurgy and gas chromatography lab in their Ford Fusion Hybrids to test and determine the country of origin of the hammer on your MP5 clone build. So 922(r) isn't going to be the point of the spear.

HOWEVER, lets say that you've made a habit of building guns without paying attention to the requirements of reporting to the ATF mothership. That's pretty serious. And if you've also managed to sell a few of these unreported weapons you are going to be in fairly deep kimchee. At that point, it's VERY likely that you are going to have raised suspicions about what other illegal shenanigans are going on. 922(r) is very likely going to be added to your list of transgressions so that Uncle Sam can extract the maximum penalty out of you.

It's disturbing, but not surprising, that there are folks at ATF who are giving you, frankly, bad direction. If you would like to privately share with me exactly who from Counsel's Office gave you a green light on the SBR builds, I'm happy to follow up.