As my earlier post suggests. Make up a Bill of sale with description Make/Model of the weapon with serial number. ID and if you want address of Seller and Purchaser with ID info recorded. This will protect both of you. I would put in a safe place as you would any other important doc's.
Say you sell to your friend. Someone steals it from him and commits a crime. YOU being the last recorded owner of the weapon will be asked about it. You can then show the Bill of Sale and they''ll go to the next person for questioning. Probably will never have to use the Bill BUT it does effect traceability and a time line of ownership. AS ALWAYS-Just my humble opinion.
"The smallest minority on earth is the individual. Those who deny individual rights cannot claim to be defenders of minorities." --author and philosopher Ayn Rand (1905-1982)