The usage of AR with body armor will remain unchanged, even if the specific values of these ratings change in the future. Using the examples of PV as they effect different body armor classes, I can easily assign PR to the specific armors in question. A more in-depth discussion of body armor will follow....
It seems appropriate that the damage applied to an item after being struck by a weapon should depend on the relative values of the weapon round's PV and the item's PR. Determining this dependence is problematic, as the balance between realism and swift game usage is rarely struck. Developing a simple rule that is easy to remember seems to be the highest priority. The best balance I've found is as follows:
To simplify: When applying damage to armor take a fraction determined by the smaller value, whichever it is, over the larger.
Applying these percentages in a reasonable amount of time will likely require the use of an electronic calculator. It seems appropriate to round to the nearest standard damage point in all damages applied, since one damage will be rounded up and another down in any assault.
After an attack has penetrated a given armor it seems appropriate to reduce that round's PV as it applies to whatever is behind that armor: Subtract that armor's PR from the attack's PV. Eventually the round will encounter more material than it can penetrate, and this reduction in PV should help reflect the loss in damage capacity. For example, a round with a PV of 6 just barely penetrates ¼-inch steel armor plate. If there is another identical plate directly behind the first the round shouldn't be able to penetrate that as well. This reduction in PV will also affect the penalties due to trauma and shock; it seems unfair for an armored character to receive the same shock as an unarmored character.
Matt fires his .38 Special (PV: 4; 2D6 SD) at Dave, who is wearing a Class IIIA Flak Jacket (PR: 6; AC: 11; 70 SDC). Matt rolls a natural 7, meaning that his round does not strike Dave directly, but hits his armor. The round has a PV lower than the jacket's PR, which means it will not penetrate through to Dave. Matt makes his unmodified 2D6 damage roll, getting 9 points. The damage applied to Dave's Flak Jacket is calculated as follows:
Damage = ( PV / PR ) * ( Unmod Dam ) = ( 4 / 6 ) * 9 SD = 6 SD
If Matt and Dave's GM is using the rules for impact and shock damage, found in The Compendium of Contemporary Weapons, Dave will take two points of damage (to his own SDC) from the impact of the round, and will feel a stinging momentary shock that causes him to lose one third of his attacks for that round.
Frustrated that his revolver did not significantly injure Dave, Matt pulls out his AR-10 Assault Rifle, loaded with Teflon-coated Armor Piercing Rounds (PV: 8; 6D6 SD). While Dave is stunned from the impact of the .38, Matt unsportingly fires a single round at Dave. Matt rolls a natural 5, but manages to strike the helpless Dave's armor. Matt rolls 6D6 for unmodified damage, coming up with 21 points. The PV of the Teflon round is higher than the PR of Dave's flak jacket, so the damage applied to the jacket is:
Damage = ( PR / PV ) * ( Unmod Dam ) = ( 6 / 8 ) * 21 SD = 15.75 SD = 16 SD
The rest of the damage, 5 SD, is applied to Dave's SDC and/or Hit Points as the round strikes through his armor.
Again, if Matt and Dave's GM is using the rules for impact and shock damage we need to determine the damage done by the shock. The round that strikes Dave through his armor acts as though it had a PV of 2, equal to the original round's PV of 8 minus the armor's PR of 6. This means that Dave will lose 1D4 attacks for that round, unless he saves vs. physical trauma/shock, rolling a 16 or higher with PE bonuses. If Dave saves, he will lose only one attack.
Matt, celebrating because his opponent is bleeding on the ground, doesn't notice that Dave has managed to save vs. shock and is bringing his .44 Magnum (PV: 6; 5D6 SD) to bear. Dave squeezes off a round towards Matt who, today, has forgotten his body armor at home. In this system, I treat unarmored humans as having a PR of 4. This means that a round fired from Dave's .44 will, if it strikes Matt, do significant damage but will also pass through leaving an exit wound. Because the bullet can still damage something behind Matt, it seems unfair to apply full damage to him from this round.
This time Dave caught Matt unawares, and rolled a 7, high enough to strike the unsuspecting target. Dave rolls 5D6 unmodified damage, coming up with a 17. The damage Matt takes from Dave's .44 is calculated as:
Damage = ( PR / PV ) * ( Unmod Dam ) = ( 4 / 6 ) * 17 SD = 11.33 SD = 11 SD
Matt, according to the shock rules in The CoCW, will lose all of his attacks for that melee round, as well as the attacks for the next four rounds. He is knocked off his feet and there is a 30% chance he will lose conciousness for 1D4 minutes. Unless Matt receives medical attention he will die! This seems extreme, considering the measly eleven points of damage, but it does satisfy a realistic sensibility.
As for the round that passed through Matt: it strikes the wall behind him, applying the remaining 6 SD and acting as a round with PV of 2.
Matt's loyal bio-android clone M.A.T.T., sensing that his master is in dire straits, uses his thermo-imager to see Dave through the wooden wall he is propped up against. The wall is about 10 mm thick. From the list of thickness multipliers this plaster has a PR eqivalent to about 2.5 mm of steel armor plate, granting a PR of 1. M.A.T.T. targets Dave with his own AR-10 (PV: 8; 6D6 SD), and fires at him through the wall. The bio-android expects the wall to take a little impact off of the bullet, but calculates that the armor piercing round will still be lethal. M.A.T.T. takes aim and rolls a natural 17, which means the round will bypass Dave's armor. The unmodified 6D6 damage roll results in a 24.
First, the round strikes the wall. The wall takes some damage, determined by:
Damage = ( PR / PV ) * ( Unmod Dam ) = ( 1 / 8 ) * 24 SD = 3 SD
The bullet passes easily through the wall, now carrying 21 points of SD, acting like a round with PV of 7. It strikes Dave (who has an effective PR of 4), passing through the meat and bone and doing damage:
Damage = ( PR / PV ) * ( Unmod Dam ) = ( 4 / 7 ) * 21 SD = 12 SD
The round now carries 9 points of SD and has an effective PV of 3. What happens to this round now is open for discussion.... Does it strike Dave's armor from the inside, or pass cleanly through (possibly striking the semi-conscious Matt)? The GM has a wealth of options, and whatever seems appropriate can be dealt with using this system.