If you are building a semi auto such as a HK 91 or a FNFAL or any foriegn weapon etc... you need a percentage to be U.S. made parts. The ATF website has the specifics. As for a receiver, if you're machining the weapon yourself you don't need a FFL. Just don't do it if you're a felon, make an auto, sell it after you're done, etc..
"...Our natural, inalienable rights are now considered to be a dispensation of government, and freedom has never been so fragile, so close to slipping from our grasp as it is at this moment.” Ronald Reagan
"Those who beat their weapons into plows will be plowing for those who don't."