The Supreme Court today decided a case involving Miranda. It is covered on the front page, but in short it involved the case of a double murder involving a shot gun. The case is depressing enough in its result: a chipping away at Miranda (though in the instance pretty minor) but that is not the point I want to make.
Let's start with the facts as Adam put them:
Late December, 1992: double murder in a home, no witnesses. Police recover six shotgun shell casings at the scene. The investigation leads police to Salinas, who had been with the deceased at a party the night before, and who owned a car similar-ish to the one seen leaving the scene. Salinas agreed to hand over his shotgun for ballistics testing and to accompany police to the station for questioning. Heck, they told him they wanted to take some pictures of him and help clear him.This isn't an usual set of facts - it happens with some frequency.
Ballistics in this case confirmed that the weapon in the defendant's possession is the murder weapon. In any case where this occurs, obviously, solving the crime will turn critically on who used the gun. Now in this case, the defendant had possession of the gun, but that doesn't prove he had the gun when the crime was committed.
Now let's imagine the facts of the case if the gun WAS registered by the defendant. First, the prosecution would be able to show that it was in fact the defendant's gun that was used in murder. A prosecutor would use the evidence in the same way they use the registration of a car in a vehicular homicide case (your car was seen hitting the victim) Couple the gun registration with the fact of possession and you have pretty good evidence about who used the gun.
Is it conclusive? No more than the evidence of the registration of a car is proof that the owner was driving it when an accident occurred But is it useful information - .absolutely. Most cases don't turn on one piece of evidence - they are instead built on many different pieces of evidence that help establish the truth.
Today Miranda decision makes a pretty good case for gun registration.