Original Attack sequence
1- Attack check, d20+attacker skill(BAB or otherwise)+agility vs 10+defender skill(BAB or otherwise)+agility
2- Block check, unmodified d20 vs Block DC(set by shield or weapon size). If failed, go to 4, else go to 3.
3- Armor check, unmodified d20 vs Armor Coverage(set by how much of the body the armor covers), if failed, go to 4, else go to 5
4- Penetration check, d20+str+AP vs armor or block hardness. Success go to 5 after halving damage
5- Deal damage

Refined version:
1- Attack check: Success goes to step 2, failure ends the attack
---1d20+attack skill+agility vs 10+defender skill+agility+block bonus
(Reasoning: yes, shields and weapons break all the time...if they take a straight hit. You don't do that when parrying properly, rather, you use them to alter the attack trajectory so that it misses. Actually blocking straight will break your arm)
2- Armor check: Success goes to step 4, failure goes to step 3
---1d20+AP vs 10+Armor Coverage.
(AP is defined by the weapon's ability to inflict damage through armor. Chopping weapons like axes, and bludgeoning weapons like maces have the highest, while slicing weapons like scimitars have the lowest. Other weapons have conditional bonuses. Daggers increase their AP rating when in a grapple, rapier and arrows increase their AP rating against chainmail or ringmail)
(Armor Coverage is purely defined by the percentage of the body the armor covers, and for self powered heavy armors like the Apparatus of Kwalish, it can go higher than full plate)
3- Armor damage: End of combat sequence
---Armor takes the weapon damage, reduced by it's hardness. Broken armor(defined as having lost at least half it's health), halves it's coverage bonus. Defender takes the same amount of damage as the armor, but any critical multiplier is reduced by the armor coverage(every 25% coverage reduces it by 1x), to a minimum of 1x.
4- Direct damage: End of combat sequence
---Defender takes weapon damage unguarded. Process as normal.

Called Shots: Modify Refined version as follows
1- Attacker takes a penalty on the check depending on size of target area relative to the defender. By default, attacks are presumed to strike for center of mass(thus, there is no called torso-shot). Penalties start at -4 and gets worse. If the attack roll misses, but remains within 5 points of the defender, it becomes a regular attack.
2- Armor coverage is altered to be based on the body part coverage. Almost all types of armor covers the head and stomach with better than 80% coverage(translating to a coverage bonus of +16).
3- Hardness is based on the armor piece used. Most leather or chain armors still use plate for helmets.

Called shots do not automatically destroy the targeted body part unless the damage would normally kill the character(as such an extensive wound is normally fatal to begin with). Inflicting half the defender's hp to the part disables the part until healing can be applied.
However, inflicting at least 1/4 to 1/10(depending on the size of the targeted part) of the defender's hp to the body part would inflict penalties based on the targeted part.