I live in Pennsylvania; handguns here can only be transferred through an FFL, though long arms can be sold privately. I have a good friend who had some rifles stolen in a break in around 12 years ago. The state troopers who showed up to take his report had a list of every firearm he’d ever bought through an FFL. They went down the list and asked him if he still had each one in his possession or if it was one that he was reporting as stolen. Make of that what you will.