Several years ago
Shotgun News had a contributing editor (who has since retired) who wrote about D-I-Y type projects for the home gunsmith. He did an article on completing 80% AR-15 receivers, and he discussed the question of marking the completed firearm. The federal law that allows you to complete an 80% receiver does not require that you mark it. However, as has been noted, you can't sell it without at least a serial number. The author of the article showed how he used transfer punches (such as from Harbor Freight) to put his name, city, state, and a serial number on his 80% receivers.
https://www.harborfreight.com/18-in...amping-set-36-pc-63676.html?_br_psugg_q=stamp
This protects you -- it's highly probable that the random police officer who sees an AR-15 with no serial number (like maybe you're in an accident) will assume it's illegal, and then the games will begin. Except in a few states, there is no requirement that you register the serial number you assign with anyone or any agency.
A few states (the usual suspects) have enacted so-called "ghost gun" laws, that include various provisions for marking and registering such firearms. Before embarking on an 80% receiver project, you should first determine whether or not your state has enacted any legislation affecting them. These laws usually use the term "ghost gun."
The other point is that, without a serial number, such guns (or at least the receivers) might have to be destroyed upon your death, since they can't be sold through an FFL. I am not a lawyer, so I won't speculate as to whether or not you would need a serial number on a "ghost gun" to sell it in a free state, where people can just sell guns without a nanny state permission slip. Federal law would, of course, apply to any interstate transfer of a firearm, even a ghost gun.
The basic rule for 80% receivers is that you are making a firearm for your own, personal use. Many authors have stated outright that a firearm made ffrom an 80% receiver can never be sold. That appears to be incorrect. You can't make it with the intention of selling it but, if you make one and five years down the road you decide you'd rather have an AR-10 instead of an AR-15, you can legally sell it. It is a completed firearm at that point (whether or not you have applied a serial number), so all federal and state laws apply.
Lastly, please keep in mind that "loose lips sink ships." I like the idea that people can make their own guns. But I don't want to see 80% receivers follow the route of bump stocks, and now maybe "pistol braces." As I mentioned, and as FrankenMauser mentioned, these are referred to as "ghost guns" and several states have already passed laws either banning them entirely or heavily regulating them. The more you talk about them, the faster the regulations and bans will spread.
Disclaimer: I am not an attorney. I didn't even stay at a Holiday Inn Express last night. To obtain a fully qualified legal opinion on this, consult an attorney in your state who specializes in firearms law.
