Who's at fault if a bullet that automatically travels forwards after the owner activates it with a button on a gun winds up piercing the body of someone else an

Who's at fault if a bullet that automatically travels forwards after the owner activates it with a button on a gun winds up piercing the body of someone else and killing them? The person who activated the bullet or the manufacturer? This isn't about AI, it's a simple dumb case "guy presses button that makes metal move. Metal crushes pedestrian". There should be no doubt about who's wrong, and it's not a moral question. Naturally if the starting was a failure of the remote start system it's the manufacturer, but still not really a moral discussion, just plain old product failure.