Watching Captain Blood I thought about that scene as well and have a different thought on it. The coals would provide almost no back pressure and would be next to impossible to ram due to the charge behind. However, have we been neglect in thinking that perhaps they were just put into the end of the muzzle of an already loaded cannon? With a ball between the charge and coals I doubt there would be nearly as much of a chance of an accidental discharge. Of course I believe there would be little to no effect.