I guess I didn’t think about the registration part. I was looking at it from a “clean” crime....where the police find the gun on the person who shot a person. They are the vast majority of murders. This whole “stealing a gun” to shoot someone is very rare unless it is a family member and the gun is in the house.
You have been watching too much TV. The vast majority of murders are never solved.
I can't find the current % of stolen guns used in murders but I did find a statistic from 1991:
"...According to the 1991 Survey of State Prison Inmates, among those inmates who possessed a handgun, 9% had acquired it through theft, and 28% had acquired it through an illegal market such as a drug dealer or fence..."
If we just take this at face value, in at least 37% of times where we find a criminal with a gun, tracing the gun would lead us to the wrong person. Now this is not the same as finding cartridge cases and tracing the gun from there, which I suspect would be even worse.
But, the real bottom line is that if we find the gun on the criminal and we have the cartridge cases, we can already match them up by forensic examination.
What this law does is point police to the wrong person in more than 1/3 of all instances.