Numerous cases concerning arms and their trafficking in the US are being decided in favor of defendants who don't roll over and plea due to the disconnect between a popular administrative convention and the actual text of the law (archived). It turns out that when the "weapon" being trafficked is an AR-15 lower receiver, the lower receiver itself doesn't qualify under the law as a "weapon" because too many of the features that the text of the law requires for it to be classes as a weapon are part of the upper receiver which is a completely different part on the AR-15.
Prosecutors and other USG bureaucrats have simply been pursuing cases through administrative convention, deciding the lower receiver itself would be the magic part they would treat as a weapon despite no single AR-15 part on its own qualifying as a weapon under the law. At present some members of the USG judiciary are already dismissing cases over prosecutor objections.
Remember kids, any plea agreement proposed by a USG prosecutor isn't ever any kind of deal.