When a cannon is fired at a unit that has just died, a shot is fired, but the projectile does not fly out of the cannon. However, it goes to reload. Given its firing rate, the effectiveness of this weapon is reduced by 50% if there is a fight. That is, it only works 100% when it works solo.
Suggestion: remove the population limit cost for the cannon so that it works like a regular turret. Or make it so that the projectile is fired anyway.