According to Wikipedia:

> A .38 caliber Charter Arms revolver registered to Abu-Jamal was found at the scene next to him with five spent shell casings.[23] Tests performed with the physical evidence verify that Faulkner was killed by a .38 caliber bullet. The extracted slugs were identified as Federal brand .38 Special +P bullets with hollow bases, which matched the shell casings in Abu-Jamal's handgun retrieved at the scene. Rifling characteristics evident on the bullet fragments extracted from Faulkner's body matched those of the handgun. Anthony L. Paul, supervisor of the Firearms Identification Unit, testified that the type of bullet was rare at the time, with only one manufacturer, though he could name two other manufacturers which produced weapons bearing the same rifling characteristics.[24] Experts testified that the bullet taken from Abu-Jamal was fired from Faulkner's service weapon. George Fassnacht, the defense's ballistics expert, did not dispute the findings of the prosecution's experts.[25]

> Amnesty International, with reference to the physical evidence, has expressed the view that "the police failed to conduct tests to ascertain whether the weapon had been fired in the immediate past ... Compounding this error, the police also failed to conduct chemical tests on Abu-Jamal's hands to find out if he had fired a gun recently."[8] In a 1995 hearing, a defense ballistics expert testified that due to Abu-Jamal's struggle with the police during his arrest, such a test would have been difficult to accomplish and, due to the gunpowder residue possibly being shaken or rubbed off, would not have been scientifically reliable.[26] A note written by coroner Dr. Paul Hoyer, who autopsied Daniel Faulkner, states that he extracted a .44 caliber bullet from Faulkner. This has led to claims that Faulkner was shot by a .44 caliber rather than a .38 caliber weapon. Hoyer admitted in 1995 that this was an "intermediate note" that was not supposed to be published, and that the note had been a "lay guess" based on his own observations, that he was not a firearms expert and that he had not received any training in weapons ballistics

So while it's pretty simple, it's once again a failure of the system to cover all their bases and make the case airtight.