They got a pic of the badguy turning in the gun, a Colt 1911 with a pearl handle (the kind Patton didn't like) but it violated the privacy clause of the turn in program, so the young lady officer who took the pic got disciplined.
The detectives caught the real suspect in a way they could use the evidence in court.
And 500+ guns were removed from the streets of The Naked City.
No mention of any other guns being crime guns.
All's well that ends well. Hey, it's TVLAND, what else is new?