The short version from ATF.Gov

Q: Is it legal to assemble a firearm from commercially available parts kits that can be purchased via internet or shotgun news?

For your information, per provisions of the Gun Control Act (GCA) of 1968, 18 U.S.C. Chapter 44, an unlicensed individual may make a “firearm” as defined in the GCA for his own personal use, but not for sale or distribution.

More here: http://www.atf.gov/firearms/faq/firearms-technology.html