As I said in my introduction, one of the major problems is that the components that are identified by a registered serial number are the cartridges and the breeches, i.e., the receivers of handguns and assault-style rifles. Moreover, most firearm components have a serial number given by the manufacturer. Since only the receiver is considered an identifiable component of a firearm, it becomes easy to produce this component at home either using polymer and a 3D printer or by retooling a piece of metal with a machine.
Once a person has done that, he or she can buy the other components individually and there will be no checks. There are virtually no checks in the United States.
There is even an industry that manufactures “polymer 80” firearms, so‑called because the weapons aren't quite finished; you have to assemble them and makes some holes in the receiver. These firearms are easy to produce in the United States. Obviously, these firearms are coming in from the United States and we have no control over what is done in that country. We can, however, reinforce our controls, not only at the border but also with Canada Post, etc.
We have to check what people are ordering. If, for example, a person is ordering lots of barrels for handguns or something similar, we have to be able to identify the components so that they are included in the definition of a firearm. That would be very important. A firearm is not simply composed of a grip or a receiver, there's also the barrel, the breech...