A police officer on the street does not have access to a computer system in which can verify the registered owner of a weapon in an instant.
True. No computer will tell you the "registered owner" of most weapons. If I buy a gun from my La. neighbor, Joe Blow, there is no registration required. I can sell it, and it can turn over, legally, many times, with no registration required.
But I bet a lot of moron criminals who do have a stolen gun may say something stupid or suspect in response to the question ("Well, you see, what the deal is, my uncle asked me to hold it and ......."), which may make the officer feel the need for further questioning.
It's like the questions about the car rental papers, where are you going, where are you coming from, what's that address, etc. All are aimed at sniffing out BS. They may also lead to probable cause for a search (or obtaining consent). Nothing unlawful in those questions, even if the computer won't immediately give the answer. It's just a lawful bluff.