Negatory. You need both a federal manufacturing license and you have to collect the excise tax that firearms and ammo manufacturers must charge all customers except LE and the U.S. Military. (That's why some pump shotguns and some ammo is marked LE only: To indicate the excise tax has not been paid. It has nothing to do with the products themselves.)
Even building from completed virgin receivers is iffy. While the receiver's manufacturer has paid excise tax on it, it's only for the value of the receiver. When you buy the other parts, no excise tax has been paid on them, and so the government considers this cheating them out of some of their due.
Remember, BATF is (and was designed to be) a tax collection agency. It's the only one with its own SWAT team, though.
The odds are, if ya build a coupla guns for some buddies, the BATF will ignore you, but when strangers start showing up with cash in hand, better put on yr vest and call Randy Weaver's lawyers.