Firearms manufacturing is a serious business with significant liability and legal implications. Are you sure you want to go there?
An occasional sale by an individual to another qualified individual is not prohibited, but manufacturing firearms primarily for sale is quite clearly prohibited by the BATFE.
If you build a rifle from purchased and registered parts and wish to sell it, you are within your rights. Build many and you may find a room at the gray bar hotel.
The caliber of the man counts more than the caliber of his gun.