Designers of roleplaying games who are interested in learning how the real world works, and not just studying other people’s stories and games, usually put a lot of thought into the combat mechanics. One old argument is about how to handle the performance of armour. Fairly early on (sometime in the 1970s or 1980s?), the idea of a damage roll was combined with the idea that armour could provide a penalty to damage. However, this tends to bother people whose archetypical combat involves modern firearms and armoured vehicles or kevlar body armour.
Bullets and shells have a very predictable ability to penetrate armour, and modern industrialized, standardized-tested armour has a very predictable ability to resist it, and the damage-roll-minus-armour model tends to let some things get through which should be stopped. While sometimes this can be abstracted away (“eh, maybe those few points of damage represent bruising”) other times that is difficult to justify (“did the shell explode inside the tank or outside? Did the Deathly Dagger of Draining touch his flesh or not?”) One solution to this is to treat both penetration and resistance as more or less fixed, then generate the effect of the wound based on their interaction. GURPS fans often refer to this as armour-as-dice, because armour can be treated as reducing the predictable number of damage dice which the attacker rolls instead of the variable results of that roll.
However, models which treat penetration and resistance to penetration as more-or-less fixed tend to make people who are more interested in combat with hand weapons uncomfortable. In this post, I would like to explore what we know about how much the ability of hand-made armour to resist weapons can vary, even within a given piece of a known form and quality. If you want, you can skip to where I sum up.