The incident happened at the Erwin Center in Gastonia.
Parent Ashely Poarch told ABC11 affiliate WSOC that she captured the gunfire and the moments that followed while she was livestreaming the game.
"You could hear the gun. The whole place shook," she said. "We just sat still we froze."
The bullet struck the gym floor and left a hole.
Police said the gun went off accidentally after falling out of the pocket of a coach who was there for another game.
45-year-old Wallace Montgomery is now facing weapons charges, according to police.