Since it's a realism discussion : add weapon knockdown to all weapons.
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Also being struck in the head is more likely to flip out and cause you to fall over than being stuck in the legs or feet.
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It could be that if you are moving and get hit with a full power swing in the head there is a 50% chance it will knock you over so long as it knocks off 10%> of your health.
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You know what that's not a half bad idea for a joke. I did however think of something worth doing though. Make knock down only work on head shots. Only the Mauls/mallets would have a chance of knocking you onto your arse with a body shot + the pike, that thing should have knock down.
You're right that idea is not bad.
However I just want to point out that IF you managed to knock somebody down with lets say a mace hit in the head, that would be because of causing unconciousness, not the power of the strike knocking somebody to the floor
On the other hand, not ALL weapons should be able ot achieve that. Swords for example are just not heavy enough to knock somebody wearing a helmet unconcious. Without a helmet... well you know, he wouldn't be getting up again after such injury anyways