Have you thought about armor classes translating to % damage reduction depending on the type of bullet and armor class
For example (here youd have to change pen values from a flat number to a 0-10 scale. So instead of 30 pen, you get 8/10 pen for example, and every caliber would have thesame /10 scale)
Paca: lvl 2. Fort: lvl 6
9x18 4pen round 60 damage
9x18 is a weak penning round, so paca takes away 40% damage flat
Whilst fort takes away 90% damage flat
But a 545x39 8 pen 50 damage round would be for example:
Paca: 5%reduction in damage
Fort: 50% reduction because of the round
And then you calculate the pen with it
So on top of the damage reduction caliber causes, you get the pen value the bullet has, to cause additional damage reductions
9*18
Paca: would leave you with 24 damage, but because the 4 pen, you get perhaps a x0.4 modifier, putting the final damage at 9.6 through a paca. Taking about 9 shots of 9x18 through a paca in the chest to kill
fort: you get the 6 damage the round would do. Again times .4 and you get 2.4 damage to the chest per shot
545x39
Paca: You get left with 47.5 damage per shot. And since its an 8/10 pen bullet, you multiply by .8, leaving you with a final 38 damage.
Fort: Here youre left with 25 damage per shot. And with the bullet having the .8 multiplier, the final damage would be 20 damage
This is just an example of a system i think would be very good, but i am not sure about how it would be in practise.