Oh, and if you build it yourself (as this person has done), no tax need be paid, no serial # need be applied, and all perfectly legal provided you have not been restricted from purchasing a weapon through normal channels. That is, a felon making a weapon is now a felon-in-possession, where a citizen with full rights making one has not committed a crime.

One of the things we did in my machining class was we built miniature brass cannons. They were .177 caliber (In the US that is a common size of BB). They were fully functional. You put in some gun powder, a bit of wadding, and a BB, then you could fire them by putting a match to the touch hole.

Somewhat difficult to wield in a gun fight though, not that we didn't occasionally imagine the students holding off an attack by firing a broadside from cannon of various quality :-)

It is recommended that you do apply a serial number because LEO may incorrectly conclude that the firearm is illegal if they don't see one.

Yes, but there is no requirement to do so. Nor is that serial # available anywhere to look up.