LA Forensics: The Sandwich Shop Murders
With a warrant, SID collected the .380 semiautomatic pistol from the apartment that Robinson had just rented. They also collected clothing from Robinson's room that resembled that described by the eyewitness. In addition, the police had received permission from Robinson to search a storage locker where he kept other belongings, and from there SID took several pairs of shoes to compare against the shoeprint from the store's counter.
His pistol was rushed to the SID Firearms Analysis Unit, and the shoes went to the impression evidence section. Criminalist Daniel Rubin test-fired the pistol with sample ammunition to stamp the gun's signature on the casings, and then compared them with those picked up at the crime scene.
"It we have a suspect firearm," he said, "we can determine if those bullets were fired from that particular firearm to the exclusion of any other firearm, as well as with fired cartridge cases."
He was aware that when two objects contact each other - depending on the surfaces of the objects, the relative hardness, the type of motion, and the pressure involved - the harder object (the tool) will leave a mark on the softer object. "In the case of a bullet being fired through a firearm, as it travels down the barrel, the inside surface of the barrel acts as a tool and that will leave a tool mark on the bearing surfaces of the bullet. With a cartridge case, when the cartridge is fired in the firearm, a tremendous amount of pressure is generated inside the chamber, causing the cartridge case to push very hard against the various surfaces of the firearm. That will leave a mark on the cartridge case from parts of the firearm."
Rubin was able to prove, with a microscopic side-by-side comparison, that the same gun had fired the three bullets removed from the victims, and that it had been Robinson's gun. But he could not prove who had shot it.