You can buy kits with 80% lowers that are not yet considered a gun.
If you have a drill press or a steady hand, you can drill out the other 20%, and get a gun with no check. It also does not have to have a serial number.
So buy an 80% lower, finish it, and build a complete firearm with off the shelf parts.
It is legal according to ATF to make your own firearm for your own personal use, as long as it is not full auto, and perhaps some other restrictions.
This is why people are trying to use 3-D printing to make guns. The one guy so famous on line had to get a license for manufacture since he intends to give or sell them to others.
I have looked into it a bit. You can buy an 80% lower with a template of where to drill it out.
Those with the right equipment can use a CNC machine to drill out the parts from a solid block of aluminum, steel, or whatever metal meets the specs.
There are places online where you can buy 80% lowers for ARs and 1911s and probably other make and model of guns.
I don't have the equipment to do this, but am intrigued by the idea, so have looked into it a bit.
There are sites that have freely shared CNC plans for all kinds of firearms. If you had the right equipment and supplies you could build all the firearms you wanted.