As usual, here at AMT, need to preface this with the fact that we are not lawyers, we do not work for the ATF, DO NOT consider this legal advice.
If you’re putting your own serial number on an 80 lower, these are the things you must include:
Your personal name: manufacturer
City and state of manufacture
Model: anything you want
Caliber: may be “multi”
Serial Number: any combo of letters and numbers you’d like
You lower must be engraved or stamped into the lower receiver to a minimum depth of .003. VERY IMPORTANT PEOPLE. Don’t be dumb and skip over this.
Be sure to document the information you use to engrave your 80 lower. A photo would be best. That way if anything happens to your AR you have the documentation to show what you did. Yea, they’ll ask you for documentation. If you don’t want to give it to them, hey, that’s for you to decide and depending on the scenario. But you should be locked tight with everything you need legally.
For all you California residents, if you built an 80% lower receiver after July 1, 2018, you have a little more work to do. You must FIRST apply to the California DOJ for a serial number. This requires the necessary paperwork and fee. The state of California will then assign you a serial number that you are REQUIRED to put onto your lower. You cannot choose your own, you must use what the government provides. Failing to do so will result in penalty from the state. Refer to the proper state authorities if you have any questions or concerns.