Cannonballs kill people through force and not through explosion.
When the heavy cannonball hits someone at a certain speed that force of the cannonball is what kills the person or at least severely injures them.
A cannon ball works to destroy things or kill people by the force of the cannon ball.
When a cannon ball is released from a cannon it has a huge amount of force and because they are so heavy the cannon ball can easily destroy something such as a car, vehicle, house, other building and even have enough force to knock someone or a crowd of people over and kill them.
The force of the cannon ball is what causes death or injury and not an explosion.
The cannon ball is just a heavy metal ball and not an explosive.
The cannon ball is not like a bullet as in it does not actually fire or use gun powder to work.
It's just the tremendous amount of force that causes the cannon ball to cause injury, death and destruction.