Why do you say it is a myth?
A gun that is registered and later found to have been used in a crime is going to give some lead for a homicide detective to follow up on. It isn't a trump card but do you think investigations have never been solved by using a gun registration list?
It's a myth, because a gun registration rarely, if ever, leads investigators to the perp.
Anyone who uses a gun registered in their own name to premeditatedly commit a crime is a moron. Chances are, the perp was found through witness ID, fingerprint or DNA ID, a license plate, car description, etc.
If there's no other evidence the gun's owner was the criminal, they can easily claim the gun is missing and they hadn't noticed. Must have been lost or stolen.
Hard to prove it wasn't.
Most crime guns are not left at the scene anyway. How, exactly, will the Cops get the serial number of a murder weapon they don't have so they might match it to the registered owner?
For registrations to work, the Cops first need the gun.
Then they need ballistics to match the gun to the victim.
Then they need the registration record.
Then they need to locate the last registered owner.
Then they need to find out if the registered owner still owned it and had possession of it (not all states do permits or registrations for private sales).
If the owner is smart, he won't commit a crime with a gun registered to him. He'll find a "clean" gun with no way to trace it.
At that point, registration is 100% useless.
So many ways to see that registration is pointless.