tldr Give specific damage sources the ability to ignore part or all of the target's resistances.
might be a little complicated, since you could have:
*flat flat resistance ignore,
*percentage flat resistance reduction,
*flat percentage resistance ignore,
*and percentage percentage resistance ignore.
it'd probably be a good idea to only include one or two of these, but for the rest of this post I'll assume all four are in, and I'll represent piercing amounts as 'x/y%/z/w%' where x...ww represent the concepts listed above in order, and will assume that final effective resistance calculations will be calculated similar to the current damage formula, such that the original resist is "damage" in the formula and the pierce is the "resistance".
so for instance, if a damage source with 4/25%/0/50% penetration damages a target with 8/30% of the relevant resist, the final damage will be calculated as if the resistance was 2/15%
the 8 flat resistance is modified by the 3/25% section. and like the damage formula, the 25% pierce is applied first, followed by the flat 4 pierce, giving 8*(1-.25)-4=2. the 30% percentage resistance is effected by the 0/50% part, in the same way, giving 30*(1-.5)-0=15%. the 2 and the 15% are then used in the normal damage formula
as an example of how such a system could be useful, consider this.
currently bullets are:
regular mag- 2 physical dmg
piercing rounds mag- 5 physical dmg
currently, 'piercing' rounds are just 'better' rounds, which is fine from a gameplay standpoint. however, it doesn't really reflect 'piercing'ness. for instance, against the unarmored small biter, piercing rounds still do 150% more damage, as the 5 vs. 2 damage is unmodified. against the heavily armored Big biter, things change. the regular magazine will do 0.11 damage, while the piercing magazine does 0.2 damage. this is only about an 82% damage increase over the regular, which means despite being 'piercing', the advantage gained over regular bullets decreases as the target's armor increases, which doesn't reflect piercing at all. with an actual penetration system, the bullets might exist like this:
regular mag- 2 physical dmg, 0/0%/0/0% physical penetration
piercing rounds mag- 2 physical dmg, 5/0%/0/0% physical penetration
against small biters, piercing rounds would have no advantage over regular. this is half gameplay, half reality, since all other things equal an armor piercing round does roughly the same or even less damage than a 'regular' round against unarmored targets, due to the fact that the round doesn't change shape as much within the body, making a cleaner wound, and has a greater chance of going straight through, leaving some of the bullet's energy in the bullet and not the target.
against large biters, things change. the regular round still does .11 damage, while the piercing round now deals a relatively huge .33 damage; a 200% increase, and making the rounds valid as piercing.
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if penetration is added, negative penetration might be as well. the same concept and formula, just with negative numbers, to represent things that do very badly against armor/insulation/whatever. for instance, the final bullet types might be:
regular mag- 2 physical dmg, 0/0%/0/0% physical penetration
piercing rounds mag- 2 physical dmg, 5/0%/0/0% physical penetration
hollow point mag- 6 physical dmg, 0/-300%/0/-300% physical penetration
this would, like real hollow point rounds, be very effective against soft targets, but any armor would be effectively quadrupled. while it could cut through small biters like butter, each shot would do less than .04 damage to big biters.
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thoughts?
edit: it should probably be hardcoded that 100% resistances can never be lowered, to preserve immunities. either that or have an actual 'immune to x' attribute.