[damoncali]

It's also worth pointing out you can already buy all of the components necessary to assemble your own firearm, all without a serial number

Not exactly true. Every legal gun has one part with a serial number that defines the gun legally. Usually (always?) it's the receiver. There are some grey areas where you can get an almost finished receiver free of regulation.

[armored_mammal]

The comment was referencing the fact that you can make a gun for yourself without a serial number as long as you don't sell it or do anything with it that is defined as 'transfer.'