Under current federal policy, it’s legal to make a gun for your own personal use. You don’t need a license, and you don’t have to put a serial number on it, though you’re not allowed to make certain highly regulated types of guns without government approval.
Yet it’s not clear exactly how helpful businesses can be when it comes to supplying the parts. The usual gun laws kick in when a business sells a “firearm,” described this way in the statute books:
(A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel