A cannon requires a full crew to work properly - to carry cannonballs, load gunpowder, push the machine round to bear on its target, and so on. If one crewman is slain then. the rest can just about get by without reducing the rate of fire. If two or more crewmen are slain than the remaining crew will be unable to cope, so when it shoots the cannon must miss a whole turn before it can shoot again. This is in addition to any penalty imposed by a Misfire result.

Obviously the cannon requires at least one crewman to work it, so the machine becomes useless should they all be slain.