Here is the ruling straight from the ATF website regarding the laws surrounding homebuilding. I hope this clears up some misconceptions
Here is the pdf version of the book of firearms laws they sent me along with my C&R license:
You should read this stuff. It's straight from the horse's mouth, and it's very clear about what is and what isn't legal.
and finally here are the firearms laws under Title 44 of the US Legal Code.
Read this stuff. It's interesting and it has a direct impact on YOUR life.