Alec Baldwin failed to follow the No. 1 rule of gun safety before the fatal shooting on the set of “Rust,” a Hollywood weapons expert tells The Post. “Loaded or unloaded, a weapon never gets pointed at another human being,” said Bryan Carpenter, who heads Dark Thirty Film Services. Even on a film or TV set, he said “you never let the muzzle of a weapon cover something you don’t intend to destroy.” The prop gun had misfired twice on Oct. 16 and once the week before, according to the Los Angeles times, and union workers said the “Rust” set...