At about 0730 mountain time, a Piper PA-18 struck terrain after being damaged by gunfire near Fort Peck. The pilot and passenger were seriously injured. The pilot was conducting a coyote control flight over private rangeland. At about 40 feet agl, the passenger inadvertently discharged a semiautomatic 12-gauge shotgun three to four times, striking the right wing and aileron assembly. The airplane entered a descending turn to the right and subsequently impacted terrain in a nose-low attitude.