Trace? If the cops HAVE the gun, and it's of post-1968 mfg, they can probably go from the factory to the jobber to the dealer to the original purchaser. If the original purchaser or someone involved in the case who knows the purchaser is in the suspect pool, they're on the right track. If the original purchaser is dead or can't remember where he sold it or it was stolen, they're pretty much at a dead end. If the original purchaser sold or traded it to a dealer then the hunt is on again, but all it takes is for one purchaser to be dead, have no memory/gun stolen to end the hunt.
If they DON'T have the gun, what's to trace? As far as I know, there is no national archive of fired bullets from each gun manufactured since 1968 that a bullet can be matched to. And if there was, wear and tear through firing and cleaning or a barrel change would make a positive match improbable.
In otherwords, for all practical purposes and discounting a miracle, 'tracing' is hollywood fiction.