Robart has a teeny gun, but it fires a lot of times. It can so fully shoot through people with tiny laser bullets that it can turn a person into a serviceable screen door.
Robart is just about the lowest downest meanest orniest son of a robot producing facility you ever visually sensed and processed understanding of.
Robart went to live in town when Techs had taken over as marshal. He was given the marshal's badge by Techs when Techs left Mars. He sold the badge to Steve 3PO in order to buy Error Message Jane her boots back, after she hocked them to have money to enter a poker game. During the poker game, she won the badge from Steve.
After Jane gave Sparks the badge back, Robart and Jane left Mars to go to the poker tournament.