The laws concerning the private building of non-NFA firearms are well documented on F-Troop's website. Until very recently, F-Troop was a part of the Treasury Department because their purpose and function was to collect taxes (yes, that's right, 80 men, women, and children in Waco were killed over taxes and it was never proved they owed them). Most of the firearms manufacturing laws are based on tax liability.
So, to net it out, because the Treasury Department's jurisdiction is defined by (a) taxes, and (b) interstate commerce, you can build your own non-NFA firearms (which incur special taxes) to your heart's content, as long as you do not engage in the business of building them for sale. But, be sure to read the appropriate laws on the website.