"There was a bang, and then a gentleman screamed," a witness said. "I saw a man bleeding from the lower right leg."
Witnesses told local media the man was shot accidentally by another attendee at the Tanner Gun Show on Saturday afternoon.
The Adams County Sheriff's Office said a vendor "had a gun discharge and strike another employee", according to The Denver Post.
One witness told CBS Denver: "There was a bang, and then a gentleman screamed. I saw a man bleeding from the lower right leg."
The witness, who did not want to be identified, added: "Someone negligently discharged a firearm, and it struck another person in the leg.
"There had been somebody that already applied a belt, as a makeshift tourniquet. It was ineffective. I applied a tourniquet that someone handed me."