Funny that, I have a few "ghost guns", they have a serial number, they were, probably, sold once or twice through an FFL, but nobody knows I own them. Why you ask, well, many of the used guns I buy, I am not asked to sign a bill of sale, and in many cases, the sellers also acquired the weapons in a private sale. And, to top it all off, if ever I am asked for their serial numbers, "legally" or not, I am sure not to comply.