The utterly illogical and pointless nature of the US gun laws can be illustrated with this simple example. Let’s take a gun owner who has an AR15 rifle.
Since barrels are available in different lengths, the owner pays a $200 excise tax to the BATF, waits 6-12 months and finally gets permission to use a barrel shorter than 16 inches. Here’s how a 14.5″ barrel looks on a rifle. The difference of 1.5″ inches is legally significant, though it makes only a minor impact on the actual performance and handling.
Now, imagine that this person owns more two nearly identical AR15, one of them registered as a short-barreled rifle, and takes both to the range. For cleaning following the range time, both guns are disassembled.
If the owner accidentally puts the 16″ barrel on the registered lower and the 14.5″ barrel on the unregistered lower, he commits are felony “manufacturing of an unregistered short barreled rifle (SBR)” which is punishable by up to 10 years in prison and up to a $10,000 fine. Note that there’s zero change in the actual functionality of the guns: the lower receivers are identical, and the upper receivers are still in possession of the same person who already paid for the “privilege” of using a shorter barrel.
The punishment for the accidental swapping of very similar uppers is harsher than most penalties for forcible rape, armed robbery and murder! How does that make any sense whatsoever?