The Truth About Guns has an interesting article on a little known law that allows individuals to build their own guns without serial numbers if the gun is 80% or less complete. There are some stipulations:

The machine work must be done by the end-user—no contracting it out or having a friend do it. The finished firearm can not be transferred in ANY manner. No willing it to your kids. No giving it to a friend. If you no longer want it, after your inheritors pry it from your cold, dead hands, it MUST be destroyed.

These rules seem arbitrary and silly.