You can request permission to build custom rifles and handguns from the ATF, your state has to be a willing participant however. Here in MD they will generally grant you the blessing needed for ATF approval.You donot need an FFL to do this.

Selling the gunis another issue. According to law you can sell (at least in MD now, I have to speak from what I know) a long gun as a private sale without an FFL. Thecatch is that you can not build or manufactureguns with the intent to sell them without the proper ATF blessing, which would include the FFL. A gray area I guess. In short with permission you can build yourself one but you can not manufacture for sale without proper ATF approval.