What about a damage saving throw with armor being a bonus.
This could be with armor ranging from +1 to + 12 Plus enhancements
This just off the top of my head
So the details can easily be worked.
Critcal success no damage
success 1/4 damage
failure 1/2 damage
Critical failure full damage
DC = 10 + damage/2
I have not extended these numbers out.
TAC would be used to hit.
critical hits do more damage and more likely to penetrate.