How does armor work?

In an SS, I see that someone with armor have 100 HP as well as 100 AC, does that mean you have the equivalent of 200 HP or something? Also, how much bullet damage will it cut down, as well as knife? Can it prevent you from getting 1 hit KOed by shovel and axe? What weapons can the bulletproof helmet prevent getting a one-hit headshot from?

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