Agreed, as of Jan 1st 2026 here in Maine they are required. Another law that is half baked. As to my limited knowledge, you need to put a serial number on them but there is no requirement to file it with anyone. Lots of good that does.You can legally build a firearm for personal use at the federal level without needing a serial number. However, if you plan to sell or distribute it commercially, you must ensure it’s properly marked.