At the Dixie Gun and Knife Show in Raleigh, a 12-gauge shotgun discharged as its owner unzipped its case for a law enforcement officer to check at a security entrance, injuring three people, state agriculture department spokesman Brian Long said.
Two bystanders were hit by shotgun pellets and taken to a hospital and a retired deputy sheriff suffered a slight hand injury.
The shotgun's owner, Gary Wilson, 36, brought the weapon to the show to find a private buyer, Mr Long said. Sheriff Donnie Harrison said it was too early to say whether he might be charged but the incident appeared to be an accident.
The North Carolina show, held at the state fairgrounds, is a quarterly event that usually draws thousands of people. The two-day show shut down early on Saturday but is reopening.
In Indianapolis, police said a 54-year-old man was injured when he accidentally shot himself while leaving a gun show.
Emory Cozee was loading his .45 calibre semi-automatic when he shot himself in the hand as he was leaving the Indy 1500 Gun and Knife Show at the state fairgrounds. Loaded personal weapons are not allowed inside the show.
Mr Cozee, of Indianapolis, was taken to hospital for treatment. Police say the shooting was accidental and no charges would be filed.
In Ohio, a gun dealer in Medina was checking out a semi-automatic handgun he had bought when he accidentally pulled the trigger, injuring his friend, police said. The gun's magazine had been removed from the firearm, but one round remained in the chamber.
Police chief Pat Berarducci said it appeared the bullet ricocheted off the floor and struck the friend in the arm and leg. The man was taken by helicopter to a hospital 30 miles north in Cleveland.