The question of "do I have to put a serial number" on my home built AK or other firearm has been coming up a lot recently.
Here is the law as I understand it with support below:
1. It is not required that you put the build location or a serial number on a firearm that you build at home.
2. You can not build a firearm at home with the intent to immediately sell it.
3. Prudence may dictate that you do want to place a serial number on the firearm as LEOs may not understand and think that you have obliterated a serial number which is illegal under both Federal and California law. I recommend your initials and a number - something like Redwood City, CA - GH-0001.
4. If years later you decide you want to sell your homebuilt rifle to someone else, you can, but it will require that it have a serial number and place of manufacture per Federal law.
5. Please be careful about the California definition of "zip gun" in PC 12020
. Building a firearm that is the same design as a firearm that has been or is being sold commercially is safe but there are large and dangerous grey areas for designs that are not already in commerce. Building an AK, FAL, or AR is not a problem under the zip gun definition however.
6. DOJ BoF has opined that private individuals can manufacture non rostered handguns. However, I'm not 100% confident of their legal reasoning so do be careful in that arena.
7. Always be mindful that constructive possession does apply to machine gun parts. Never have a receiver that can accept FA FCGs and generally don't have FA FCG parts around period. Searching the forum will give you more information that at least one Calgunner has gotten into Federal hot water over FA fire control parts.
For clarification on the serial number issue please refer to this ATF letter:
Hopefully this clears much of this area up.