Conclusion: armour has Durability a (a flat value), as well as melee/bullet/energy resistance b/c/d (percentages). As a decreases, calculate (current durability/total durability - e.g. 75% means the armour is 75% intact), and then apply that percentage to the resistances. At 75% durability (e.g. 75/100), the resistance values are 75% of what they usually are (so bullet resistance 60 turns to bullet resistance 45); at 10% durability, they only protect against 10% of their original (so melee resistance 25 turns to melee resistance... 2.5).
When an attack is received, b/c/d% of it is resisted, and the user takes the unresisted damage. The entire damage from the attack, including the resisted damage, is transferred to the durability pool.
Some examples ahead, you can stop reading now if you like.
Example: An ERT trooper's armour might have 60/60/60 resistance, as well as 500 durability. Someone shoots the trooper with a revolver, which does 60 brute damage (bullet). 60% of this is resisted, so the trooper takes 24 damage; the armour is reduced to 440/500, or 88% durability. Its new values are 53/53/53 (well, 52.8).
The trooper is hit with another shot. It does 60 brute - of which only 53% (31.8, or 32) damage is resisted; now they take 28 damage, and the armour is reduced to 380/500, or 76%. By now, an unarmoured person would be in crit. The new armour values are 46/46/46, and the trooper has taken 52 damage, and is bleeding.
The trooper is hit with another shot. 46% of this is resisted, so the armour soaks 28 damage - but the other 32 transfers to the trooper, who has now taken 84 damage. The armour is now at 320/500, or 64% (see how it's going down by 12% each time, since 60 is 12% of 500?); its new values are 38/38/38, down from 60/60/60.
The trooper is hit with another shot. 38% of this is resisted, so the armour soaks 23 damage; the trooper takes 37 damage, and falls into crit. The armour is now at 240/500, or 52%, with new resistance values of 31/31/31.
The ERT trooper took four shots to take down, as opposed to the two from an unarmoured target. The values there are just for explanation; the actual durability would be much less, since armour falling apart is fun - it also means if you hurt someone in a hardsuit (high durability, low resistance) enough, they're not protected from space! Ideally, armour would fall apart at about the same point where the target reaches low HP; so even if they win the fight or get healed, they're not clear to get into another encounter until they fix their armour.
The same principle could apply to shields; instead of a block chance of 50%, give them a flat 50/50/50 resistance (or, say, 25/25/100 for the eshield), and ~100 durability. Damage that passes the shield hits the armour. Doesn't that mean you can take out an eshield with five energy gun shots? Yes. Yes, it does.
... okay, how about that same ERT trooper in the surprisingly durable armour, but versus an energy sword (32 brute/melee, AP, say, 40)?
First hit ignores 40 of the resistance, total 20% resisted, so the ERT trooper takes 26 damage (6 is saved). The armour's durability is reduced by 72, for a total of 428. Its new resistances are 51/51/51. The next hit ignores 40 of that... so the ERT trooper only resists 11%, taking 28 damage (4 is saved, and they've taken 54 so far). The armour's durability is reduced by 72, for a total of 356/500, or 71%. Its new resistances are 42/42/42. The next hit ignores 40 of that, total resisted 2%, or 1 damage; the trooper takes 31 damage (now they've taken 85 - most people would've been hit for 96... the armour doesn't do much here), the armour takes 72. Its new durability is 284/500, or 57%, and the new resistances, 34/34/34. Subsequent hits ignore armour entirely, but continue to drop its durability.
It takes four hits to crit the ERT trooper with an energy sword. It takes four hits to crit anyone else with an energy sword, too, since 40 AP is really good.
Damage if they had a 50/50/50 shield too, and were being shot at with a stetchkin:
50% of 25 = 12.5 (13) damage is saved. The shield takes 25 durability damage, total 75 (new resistances 37.5%), and 12 damage hits the armour...
60% of which is resisted (7). The armour takes 12 durability damage, total 488 (97%, new resistances 58/58/58), and the target takes 5 damage, out of an original 25.
Doesn't that mean you can magdump someone with a shield and good armour (e.g. the HoS) and you might not come out on top? Yes. Yes, it does. You have to play smart now, rather than .357 your way to victory.
Actually, let's examine that. Hypothetical HoS values of 50/50/50, durability 200, and a shield of 50/50/50, durability 100 (a real shield should probably be more durable than this, as you'll see).
The first shot drops the shield to 40/40% durability (20/20/20) as 30 damage is soaked. The 30 damage transfers to the armour; half is resisted, so the HoS takes 15 damage. The new armour durability is 170 (85%), so the new values are 43/43/43. So far, so good.
The next shot kills the shield, removing its protection entirely. Just 40 of the 60 damage can hit the shield, so let's analyse that. The 40 damage is turned into 32 damage, due to the 20% resistance. The shield no longer functions, since its durability value is 0. But don't forget the other 20 damage!
The HoS's trenchcoat takes 52 damage; 43% (22) of this is resisted, so the HoS takes 30 damage (total damage taken: 45). The armour drops to 118/200, or 59%, and the new resistances become 30/30/30. The HoS has now taken two shots from a revolver, for 45 damage; most people would be in crit around this point, at 120.
The next shot ignores the shield (durability 0/100, resistances 0/0/0 - if the HoS lives, they should really repair it), and just 30% (18) of the damage is resisted - the HoS takes 42 damage (total 87 - unarmoured people would've taken 180 by now, and the current HoS would've taken either 0, 42, 84 or 126, depending on RNG). The HoS's armour now has durability 58/200, resistance 15/15/15. Another shot will both ruin the armour and crit the HoS - so it takes four shots to take down this particular HoS with their shield. In contrast, the current HoS-with-shield resists a flat 30%, as well as blocking half the shots they take; each shot does 42 damage, it takes three to crit, assume either one or two is blocked, so it takes between four and five shots. The shield durability should probably be higher - 150, maybe.
