“I suspect that if they have that microstamp, they could track the firearm anyway, without registration, thru the dealer network, assuming that it was not transferred illegally, or thru a private transaction.”
You got it. If the microstamping of ammunition is to be effective, then the firearm must also be registered. Otherwise, there is no mechanism to link the ammunition to the gun.
But how would gun registration make this work? Most crime guns are stolen. So the police would trace the fired casing back to the original owner whose gun was stolen. So what? Still doesn’t tell you who the criminal was.