There should never be a real gun on a movie set except in the hands of the security people who are held fully to account if somebody gets shot or dies on their watch. It is a needless risk that verifiably kills people.
Alec Baldwin starred in the movie The Hunt for Red October, about a Russian submarine that departed Murmansk, a Soviet Naval port. On the set of a movie, in October, he shoots and kills a woman that grew up in Murmansk, where her father served in the Soviet Navy. Bizarre to say the least...
As much as I think the guy's an asshole, I can't fault him for what happened. He was handed what he was told was an unloaded "cold" gun. He was practicing the scene where he had to cross draw the pistol from a holster and aim it at the camera. The hammer was accidentally released but it shouldn't have mattered if the gun was unloaded as he was told. Just a bad deal all around.
It sounds to me that the prosecutor used the defense's modified-gun report as an excuse to try to back out gracefully from this prosecution that seemed to be a stretch from the beginning. . Isn't it odd that a defense expert would find something crucial that the experts at the FBI lab missed? The whole modified-gun theory seems fishy.