McGrath, as you suggest crossbows, I guess you have not yet seen a bolt shot from a crossbow go through a 3mm thick piece of high qualitiy steel (I have seen that - very impressing)...

Shortly after the invention of the crossbow, the then-current pope said: this weapon will lead to mankind exstinguishing itself.
It was the first weapon with which you could kill a knight from a distance, even if the knight wore full armour.