Back in the day I read a mechanic for "armour saves" in the (almost playable) Mutant Chronicles RPG. Now, MC used a hit location and distributed armour system very similar to Dark Heresy, but added in a mechanic for rolling to determine how effective armour was: If you rolled less that the armour points, then damage was reduced by the roll. If your rolled more, then… I forget if you got the full armour value or nothing.
For GM's that are concerned that the inevitable power creep in personal protection will make the players jaded about low-end challenges, an armour save system may be of interest. Players may be cavalier about threats that they are sure to survive, but they are a lot more concerned about threats they'll
just probably
survive.
So, for the purpose of this excercise, let's assume that a mechanic that undermines the reliability of armour in the game is desireable .
Armour Save
A character that is hit on a location where he has armour has a chance that the armour will absorb some of the damage. Roll 1d10 and compare the result with the effective AP on that location: Remember to double AP for normal armour vs. primitve weapons, halve AP for primitve armour vs. normal weapons, and apply the PEN of the weapon. If the roll is equal to or higher than the effective AP value then the armour provides its full effective AP value in protection: Reduce the damage by this amount. If the roll is less than the effective AP value then the armour protection equal to the roll of the dice.
Ok, so the main selling point is that armour does not provide a static protection value. Having armour isn't pointless ( some protection is better than no protection). A character with effective AP 6 has a 50-50 chance of getting the full benefits, but there is always the chance that a high damage roll from an otherwise insignificant weapon (e.g. an unarmed attack, or a knife) may coincide with a low armour save roll. Also, even a low armour save roll will provide some protection.
The most obvious down side is that introducing extra dice rolls also make combat slower to resolve. So maybe you don't need to roll for each time a character is hit: Maybe it is sufficient to make an armour save against attacks that roll a 10 for damage (Righteous Fury not-withstanding), or if the damage scored is at least 10. After all, the point isn't to bog the game down with minutiae, but to keep the players on their toes by making combat less predictable.
-K