The question is how to model this in a simple way so a single die roll can be enough to resolve damage during combat and yet still be realistic enough to represent armor and cover.
Here are the design goals:
- represent the stopping power of cover (walls, cars, other people?)
- represent deformation as the bullet penetrates cover
- represent the stopping power of armor
- represent deformation as the bullet penetrates armor
- represent the actual damage done to the target
Now the issue I'm seeing is that simple hit point damage isn't good enough to represent the penetrating power of a bullet. A bullet or buckshot can do a lot of damage to tissue, but be terrible at breaking through armor. Bullets may be very good at going through level III or IV armor, but not be that great at stopping the target dead on its track.
The complexity resides in that simply raising the HP damage value may turn a lethal round with very poor AP performance into a very effective at defeating level IV armor. Which might not be the case. On the other hand "small damage" rounds may be effective and defeating level III armor, but cause too much damage to the target. More damage than the round would really do to an unarmored target, leading to excessive lethality in the game.
Also, as bullets go through objects they don't only lose momentum, they become deformed. So a bullet going through something covering the target will be less effective at penetrating the targets armor. This requires a quick way to represent bullet deformation to grant the target some advantage to taking some cover.