> You can feel free to make a stock, a trigger, a pistol grip, etc., but just don't make a receiver. On some guns there's a lower and upper receiver, in which case you can make the lower without the license. Make the upper without a license and you're in trouble.

This is incorrect, assuming you're in the US and no weird state laws are relevant. In general, it's perfectly legal to make your own guns from scratch. A license is only needed if you plan to manufacture guns for sale.

http://www.atf.gov/firearms/faq/general.html#gca-manufacturi...