I was talking about an unserialized gun. You can't manufacture one without a serial and then sell it or even give it away. You can manufacture unserialized guns for your own use, perfectly legal.

I know of no loophole in the laws that would permit you to stick some random number on a gun and call it a serial number and make it legal to transfer a home built firearm. To issue a serial on a gun, I am pretty darn certain you must be licensed to manufacture firearms. If anyone could issue a legal serial, people would be tattooing home made guns with random numbers and make them legal to transfer. Not happening.

With a manufacturing license, yes an individual could make a firearm and serialize it. In fact you would be required to serialize it for any use other than your own or you would loose your license and probably spend quite a time in prison. By serilizing a firearm, it would require the firearm to be registered with the ATF which would begin the paper trail for it. After that the initial purchaser(recipient) can simply give or sell it without doing any paperwork or registering the sale/gift. At least in most states. This area gets fuzzy at the state level though. And the laws differ between long guns and pistols. And even within long guns there are variants where laws differ (SBR for example).

I have not verified but I believe that the restriction on transferring unserialized firearms also includes inheritance. That means when I die, any non-serialized firearms I have made are technically not part of my estate that would be transferred to my inheritors. They would have to be turned over to ATF for disposal. I am also pretty sure they can't be placed into a trust either in an attempt to circumvent those restrictions. Again, I have not verified these assumptions with a lawyer.