After WWI Frank Richards realized he had a special talent. He could absorb a cannonball to the stomach without taking serious injuries. 

Frank was famous for letting people punch him in the stomach, he even once invited heavyweight world champions Jess Willard and Jack Dempsey to take a crack at him. 

Including fists, Richards also invented people to jump on his stomach but eventually, he grew tired of the charade and upped the ante. That's when he decided he would shoot cannonballs at his stomach for fun.