[Nevercast] - Indepth Setting Discussion

[horomancer]

It's all confusing. If you punch someone you are exerting more Force than if you shot them with a gun since Force = Mass x Acceleration. A bullet is constantly decelerating and has much less mass than a fist (let alone the whole body weight). However the Kinetic energy of a bullet(KE= 1/2 Mass x Velocity ^2) is order of magnitudes larger than a punch, but has much less momentum, assuming the punch is thrown with the entire weight of the body (Momentum = Mass x Velocity).
A target struck by an object with high Kinetic energy will be damaged differently than one struck with a high momentum. The short of it being body armour is great at stopping high energy attacks, which characterized by high penetration  and not so great at stopping high momentum attacks. You will go from dieing horribly to just hurting if shot in the chest by rifle when wearing a bullet proof vest. That same  vest will do little to dampen the trauma inflected by a baseball bat, though the bat is less dangerous to begin with.
My physics isn't as strong as it use to be, but I believe this is due to the Impulse of Force which is defined as Imp= F delta(t) = Mass delta(velocity)
http://hyperphysics.phy-astr.gsu.edu/hbase/impulse.html#c2
The strike from a heavy momentum object creates a shockwave, which in turn travels though dense armour (like trauma plates) well and passes with less dampening into soft squishy bits.  Large volumes of soft armour, like Styrofoam, are used since the padding effect serves only to increase the time it takes for the concrete to meet your head, robbing it of Force. How Pressure comes into play is beyond me, as well as the effects of Elastic and Inelastic collisions, though i recall hearing once that a strong punch was equal parts thrust and snapping the fist back after impact. The theory being the less time the fist and target where in connection the greater the Impulse of force as well as a better transfer of Kinetic energy.

For your game, just say most armour built for modern combat has a really high defense for Penetrating and crappy defense for Bludgeoning. If a Martial arts master targets a weak spot with his weapon then the armour wouldn't come into play and the technicals can be cleanly skirted. If a MAM struck a strong portion of the armour with his special sword, the penetrating effect would be negated by the plate spreading the force over a large area, but the shock wave produced would be the same as if the MAM had punched him, making it a bludgeoning attack.