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Weapons would have 4 primary stats (aside from weight, ammo capacity, size, etc.): accuracy, penetration, lethality, and damage. Accuracy would serve as a cap to the potential modifier you could apply to an attack roll, since a weapon is only as good as its wielder and a wielder is only as good as its weapon. Penetration represents the raw penetrative capabilities of the weapon. Lethality represents how nasty an injury the weapon leaves, be it through deformation, fragmentation, yawing, or just leaving a really wide cut. Finally, damage represents how much raw physical damage a weapon can do the superstructure of a creature or object. I'll explain what those last three do in the next paragraph.
When you attack a target, you'd compare the degree of success on the attack roll to basic table, to determine the maximum grade of injury possible. You'd then compare penetration minus any cover or armor modifiers to the target's size, with each interval of the target's size being another grade of injury higher (so a size six target compared to a 20 penetration weapon would potentially receive a Lethal injury, assuming you rolled a high enough attack). You'd add lethality to this value, which could potentially increase the grade of the injury past the maximum you rolled. Damage really wouldn't be worth bothering with unless you were trying to destroy an object for the most part and would just deplete structural health.