1) You can try building ONLY if you have a valid LTC/FID. If you don't, you're a criminal and should go to prison.
2) You can build but you CANNOT sell it or transfer it to ANYONE! If you do, you're a criminal and should go to prison.
3) If you build it, you MUST register in EF-10 within 7 days of completion. For serial number, enter NSN. If you do not register, you're a criminal and should go to prison.
Serial numbers are used to track a gun's ownership chain from the initial FFL all the way to the last owner. Since it is ILLEGAL to transfer the gun to anyone, the ownership chain starts and ends with you.
Just by virtue of having a homemade gun, you can be required to show your gun to police investigating crimes where a non-serialized gun was used. Let me rephrase it. If you build a gun, you give police permission to raid your home whenever there is a crime where a non-serialized gun is used.