Many street guns have been used in violent crimes including murder and have a legacy ballistic evidence trail that could get any owner in a lot of trouble.
Any gun obtained from a criminal should be sent to the police for forensic evaluation to determine it's criminal history.
On the topic of returning guns to violent criminals who hold others at gunpoint and threaten to murder innocent people in unprovoked assaults, lets just say I'm not in favor of leaving the offender in any position or condition to have a weapon of any sort returned to them.
The idea that individual guns can be matched to crimes with ballistics, is mostly mythology from the media.
If you have a gun that you *think* matches a bullet from a crime, the ballistics can give a good idea that it was the gun used.
But ballistics are not gun fingerprints. A gun does not leave a history to be traced to individual crimes. If you have good sample of bullets and brass, you can often match a model. Far more often, no match is possible.
In the whole history of the New York ballistic data base system, where they kept a data base of cases from the guns sold in New York, my understanding is that they did not solve a single case using it.
Modern firearms are even less susceptible to ballistic matches.
Even with several known examples of Glock pistols in a police shooting, it was impossible to determine which one fired a fatal bullet in a case that I recall.