Have you seen the core rules or the weapon / armor book? If you don’t have a copy I can send you review copies.
Without rewriting those, the system in the Codex has worked pretty well I think, and is a reasonable approximation of the reality.
In a super abbreviated form, here is the breakdown.
Weapon penetration depends on attack type. P and B type attacks (Piercing / Penetrating or Blunt / Bludgeon) have the best penetration (or ability to cause damage in spite of the armor). Slicing or slashing has the worst, and Chopping or cleaving is in the middle.
The armor DR is usually doubled against Chopping attacks and tripled against Slicing attacks. (This also depends on the type of armor, textile armor can be more vulnerable to slicing). Conversely Slicing attacks do the most potential extra damage against bare flesh, while Chopping attacks are in between, and Piercing or Bludgeoning attacks do the least.
This only matters if you get a critical hit though.
Any kind of solid metal armor is more or less impervious to most hand weapons, unless you are extraordinarily strong or have special skills. For example an Iron Cuirass has a DR of 9 / 18 / 27. A Milanese harness (untempered steel) has DR of 12 / 24 / 36 for the main armor and 8 / 16 / 24 for the ‘weak spots’.
A typical sword or dagger does D6 to D8 damage. Some do more but can only do Critical Hits for slashing and chopping (Falchion, Backsword, Killij). If you hit in an armored place with a dagger thrust, with D8 damage + your strength bonus, you might cause a small wound. With a critical hit in a thrust, if you used all four dice, you might punch through it and cause real harm. With a Cutting or Slashing attack you are unlikely to harm someone through an Iron Cuirass.
The damage is still variable but the armor is pretty effective, your best bet is to go around it (if possible, such as if they are wearing just the breast plate) or to grapple them, or use an armor-piercing weapon.
A roundel dagger, a military pick, an estoc or an ahelespeiss all have some enhanced armor piercing features, which basically eliminate part of the armor. For example if you were thrusting with an ahelespeiss (awl-pike) you would get a +4 AP, meaning the Iron Breastplate would only have a DR of 5, making it more likely you could hurt the wearer. You’d still need a critical hit to do real damage but you would be poking holes in the armor and making little stab wounds.
Lighter armor such as a gambeson or an arming doublet have less protection, for example a light gambeson has a DR of 3 / 6 / 3, which means it only partly protects you. An arming Jack, which is a gambeson that has little pieces of metal sewn inside it, protects at 5/ 10 / 15. The metal is good protection but there are gaps. A brigandine doublet protects at 7 / 14 / 14 – it’s much better protection though somewhat vulnerable to slicing.
This may not represent the full protective ability of armor but it’s pretty close. In theory if you did 4 Dice attacks you could pierce armor sometimes but in practice people use armor piercing weapons and / or try to go around the armor (Bypass attack in Codex rules). I think it works pretty well.
