Not technically a defensive gun use, but kid commando gets even more bonus points for using close contact weapon:
“You hit them in the back of the head with a machete?” the 911 operator asked after Braydon told the story.
“Did you see if he was bleeding at all?” the operator said.
“Yes,” Braydon said. “There is blood on my floor.”
Story here.
More cases of kids using lethal weapons defensively here.
