Some of this is random wibbling whilst I gather my thoughts, but please bear with me
 
Two of my favourite fantasy RPGs are Dragon Warriors and the old Lone Wolf gamebooks. Two things I love about the combat system in Dragon Warriors is that it separates the Attack and Defense into two stats, and it separates the idea of physical damage and punching through armour by giving each weapon an armour bypass stat (dice) and a damage stat (fixed). Its the fixed damage that bugs me as, as far as I'm concerned, my level 20 combat monster should be able to decapitate a level 1 goblin with a single swipe of his +3 big-sword of goblin slaying (without relying on rolling a critical hit)...
This is the good thing about the damage system used in Lone Wolf, IMHO, because it directly links the damage dealt to how much better your attack is to their defense, so level-20 combat monster can split skulls in a single swipe, but two evenly matched duellists may score 1-point hits off each other for a dozen rounds before one of them succumbs (or if you prefer the luck-runs-out theory of hit points, takes a fatal blow)
 
So, in a house-ruling frenzy several years ago, I melded the two systems and created a vast, Rolemaster style damage table varying weapon damage by type and degree of success, whilst keeping the armour bypass rules unchanged. A year or two later that I found the documents laying around my HDD and translated it into a simpler percentage formula, where an only-just-made-it hit got something like 20% of the normal weapon damage, and a maximum (but not critical) success did 300%. Criticals always bypassed armour and always killed.
My players tested this by picking a fight with a group of high-level NPC knight crusaders whilst still travelling to the adventure and promptly got their asses handed to them noticably quicker than usual, so it didn't get their vote despite me pointing out what would happen with they used it on a group of lower ranking creatures...:rolleyes:
 
On a side note, I very nearly stole the Armour Bypass concept for a Cyberpunk 2020 houserule during discussions about the problems with the standard CP2020 system not allowing for small, high velocity bullets penetrating armour better than big, slow rounds, but not doing as much damage. Unfortunately I picked the wrong ammunition types when I gave an example and was shouted down by the sites resident gun-bunny and asstard...:rolleyes: But I thought it would have worked nevertheless

One idea that I'm looking at re: damage houserules is using an Armor Piercing characteristic for weapons.  So a sword might have an Armor Piercing or 1 while an axe might have an Armor Piercing of 2.  Say both do 1d6 damage, and both roll a 4 versus a target with 2 points of Hardness 1 armor.  Since the sword has armor piercing 1, the same as the armor's hardness, so take 4 points of damage and subtract 2 from the armor... net 2 points of damage.  In the case of the axe, it's armor piercing trait is one higher than the armor's hardness, so the armor points are halved: 4 damage - (2 armor points/2) = 3 net damage.

I'm pretty sure that I'm not explaining this clearly and efficiently, but I think it gets the idea of the mechanic across... (which is basically lifted from the way armor piercing works in HERO).

Is this a good approach for armor and armor penetration?   I haven't thought it through too much, so I'm wondering what the drawbacks might be.

As for your cyperpunk 2020 example, while I'm not familiar with that system, you could handle the bullet situation by having, say, a 1d6 damage AP 3 bullet, that would cut through 3 points of hardness 1 armor unimpeded, but 4 points of hardness 1 armor would stop 1 point of the bullets' damage.
Your "big, slow rounds" might do 1d8 or 2d4 or 3d6 (etc) damage, but only have AP 2.  The same 3 pts of hardness 1 armor would stop (1.5 round down) 1 damage, while the 4 pt armor would stop 2 points of damage.

Adjusting the armor and damage values appropriately should be able to simulate a variety of results without _too_ much system-mechanics overhead.

For example, mail armor could be defined as Hardness 1, +1 vs edged weapons.
Another feature of this approach that I see as an advantage is that more damaging attacks remain more likely to penetrate armor and get more damage through (as opposed to divorcing armor penetration and damage into two separate stats that aren't interdependent), but it has a little more depth than simply having an Armor Points number that is simply subtracted from all incoming damage.

For my so-variant-it-might-not-be-accurate-to-call-it-d20 variant mine uses the damage scheme from Dangerous Journeys. This uses d3s and d6s, ranging in number from 1d3 for a shuriken in the hands of a novice, to 6d6 for a lance or zweihander.

In addition there is a abstract hit location system, know here as Strike Location. Exactly where one gets hit is not specified. What is specified is how much more damage is done because the location hit is more or less vital than another location.

In addition a more experience combatant does additional damage with his weapon. More d3s for a more experienced shuriken flinger, or more points for a more experienced zweihander swinger. So it's possible for an experienced landsknecht with a zweihander to do as much as 6d6+6x4 points of damage (for example) with one shot. Even on strictly average roles that will kill most people.

Of course I did have to adjust hit points. They became Constitution times six. Why times six? Because it matched how hit points work out in Dangerous Journeys. In DJ a character's Physical Trait works out to be about six times the value of a typical D&D character's physical characteristics.

It means that someone with a Con of 15 will have 90 hit points. He can take a lot of nickel and diming, but a single blow can kill him.

This system of mine goes along with an active defense. That is, the defender rolls against the attacker, high roll wins. With the defender winning ties. So hit points don't change unless a character's Con changes.