[bwhite]

You need a Federal Firearms License if you are "engaged in the business", which is clarified further as having a "principal objective of livelihood and profit through the sale or distribution of the firearms manufactured". See 18.921.a.21.A. The ATF and FBI have employed a wide range of interpretations for this clause, so an FFL might be warranted.

There are already shows and traveling folks with portable CNC setups who will mill a block of aluminum into various types of frames or AR-15 lowers. The 3d-printed issue seems quite similar to that.