This, by the way is also why testudo or any other form of defensive formation that is supposed to protect people against missile damage does not work. A simple way to fix this would be to take away the slinger's armour penetration completely and lower the value for all other units -OR- (and this is a very big 'or' and should probably be a much smaller 'and') to make armour penetration depend on range to target
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I dont know where they saw that a sling could penetrate an armor or a heavy shield,but i havend read about slingers in ancient warfare to penetrate armor or even shields, of heavy armoured units.
Of course i learned to respect them, since during my first major encounter against them, my 15 unit Macedonian army, of pikemen, hoplites, merc veteran hoplites and cav. was anihilated by an army of Rhodes half militia hoplites half slingers.
I had 50% casualties before my units come to contact with their hoplites pnly by sling fire.