The armour rating of a weapon refers to how it applies to various armoured targets.
ASSASSINATION
This information is clearly described in the occupation description of “Assassin”, but bears repeating here. When an assassin successfully assassinates a target (acquires, then gets placement by rolling a six (6) or beating the target by ten (10)), the victim's stamina is automatically reduced to zero (or less, for higher-stage assassins) regardless of how much stamina, makes the victim unconscious, and bleeding a mortal wound. This is assuming that the assassin can actually get at the victim's vitals, meaning that either the victim had no vital armour, or the assassin could punch through. When a being has been assassinated, they are recoverable for only a short time, because of the mortal wound.
FORCE
Force can sometimes have a significant part in damage inflicted. Whenever it may be applied, it does damage with the ratio of one (1) point of stamina for every 1000 Newtons of force. This is what allows large beings to inflict large amounts of damage in hand-to-hand combat. The force applied by guns can inflict damage in several situations: when the target is against an unyielding surface (like an armoured wall), when the target has no means of dealing with the impulse (like rolling with the blow), or when the referee is in a good/bad mood and feels like it.
By the way, 1000 Newtons is the force required to hold up 100 kilograms in one standard gravity (10 m/s/s).