One small note:
Might make the selection of weapons more deliberate, and is actually pretty realistic:
High penetration guns should do less damage to unarmoured targets. One of the reason the is special "manstopper ammunition" because the normal ammunition has to much penetrative power, thus a. leaves a very narrow wound in the target, which, as long as no vital organ is hit, or bone broken / shattered, and the target has a high pain resistance, is not stopping a combatant (bleeding mostly becomes only real problem when a main artery is hit, the rest can be bandaged quickly). b. The high penetrative power means more persons could potentially be hit by the bullet exiting the original target without having slowed down enough, which is why the police often uses such rounds.
Now, if you use a 0.5 HMG with AP ammunition as a amrour piercing weapon and hit a human sized target with it, I guess it doesn't matter much... a 0.5in bullet will leave an enough broad wound canal to almost guarantee some vital damage.
We have a very powerful AR in service here in switzerland. Actually to powerful against normal unarmoured targets. But, there is a weird thing with the flightpath of the bullet. Bullet tumbles for the first 100 meters, when it starts to stabilize to travel another 400 or more meters. AFAIK not really a "feature", but helps making hits against unarmoured targets more lethal as the bullet would often hit the body at a slight angle, thus getting into a spin inside the body, leaving a very nasty wound canal.
But these are exceptions. Most of the time, the better the armour penetration, the more narrow and long the projectile, and the more speed it has. Both mean a less severe wound, or overpenetration in case of vehicles.
When more penetration is achieved by increasing the energy of the porjectile beyond more speed, by increasing caliber and mass, that overpen effect might be mitigated somewhat by the larger wound (or penetration) canal and thus higher chance to hit vital spots, or shattering bones (nasty)
Coming back to your game, maybe you want to reduce the damage of high penetration weapons against unarmoured targets because of this? So the player needs to carefully choose his weapons arsenal?
EDIT: Almost forgot. A more technical explanation to why higher powered projectiles to less damage to soft targets:
Damage done could be expressed as energy transfered from the projectile to the target. A projectile with a ton of energy (which is mass times speed or something like that), that hits a soft target and leaves the target on the other side has not transfered much of its energy to the target. Aside from a mostly clean wound canal, not much damage done.
A manstopper round fired from a lower powered gun will transfer ALL of its energy on the soft target. It has been designed to deform in a way that it transfers its energy more efficiently, and thus will most probably not leave the soft target. ALL energy has been transfered to the target, which, besides a much broader wound canal because of the deformed bullet, means more secondary damage to the tissue, and shock.
This is also the reason why a shotgun CAN have the effect of hurling even a soft target back many meters. Pellets are small, but slow and built to not leave the target. if many hit, the transferred energy might be enough to hurl back a human.
Contrary to how hollywood and games often show it, the same shouldn't haben if you get shot by a highspeed round into an uprotected part of your body. Shot will go clean through, without the target being flung against the wall.
But I stop my rambling here.
