I don't know guns, but it seems like I always hear that nothing else sounds just like a gun firing a bullet, including one firing blanks. In fact, you actually hear that in a Columbo episode, but I can't think which one (Playback?).
Yes, it was PLAYBACK. Columbo fired the bullet into the box of sand. Harold asked him, "Why not just use a blank?" Columbo replied, "Doesn't sound the same. Never does." Or something like that. I wouldn't know, either.
Yeah, i agree it was a stupid move by Heyward. I can't recall, but was Columbo closing in or Heyward just getting paranoid? I know Columbo asked a sergeant to let him know if Heyward goes to his office alone so he was sniffing something out.