The dart gun is a toy of Calvin's that shoots out suction-cup darts. He once tried to shoot a horsefly that was on his mother's head. He also tried to shoot his mother in a Tracer Bullet story arc where his mother accused him of breaking a lamp.
He also once interrupted a get-together between his mother and her friends by shooting at the guests with the gun. Also, he once shot his dad, thinking that he was a monster.