The sound of gun shots isn't exactly disturbing the peace when it's at the gun range and nothing compared to the explosions over The Vault. It catches Atom's curiosity though, and he pokes his head into the cavern to watch. This was one of the safer chances to observe the local weaponry in action without worrying about an actual battle.

He's distracted by Harlock's skill enough to forget to announce himself and remains lurking in the doorway, wondering how many hours Harlock had to put into learning to shoot with either hand.