I thought I knew everything about Baldur's Gate and its Thac0 and AC system. But while playing BG1:EE again, I got confused.

I meet the friendly bountyhunter Nimbul, who is upset that he is wasting his time killing a weakling like me. I start rain my range weapons on him.

Now look at Khalid and Jaeira's rolls. Both rolled 16 (18-2). However Khalid's shot hit while Jaheira missed.

Khalid is using a bow and Jaheira is using a sling. When I look at the item descriptions, they are both considered as "missile" weapons. Shouldn't that mean that they are both countered by the same AC? Shouldn't both of them miss or hit?

Baldur's Gate Wiki page says

the game will show attacker Attack Roll + (defender Armor Class Modifiers - attacker THAC0 ) = result. Where, result + defender AC (not shown) = zero or greater will be a hit.

So when it says 18 -2, 18 is my D20 roll and (-2) is (Defender AC modifiers - Attacker THAC0). The "Result" is 16.

So the game shows Result + AC which is 16 + AC of the defender > 0

?

I think my mind stopped for today. I can't think straight. Please help an old guy here :)

Bow is a separate skill. Sling = missile weapon.

It's possible Nimbul has different armor class modifiers against blunt (sling) and piercing (arrow) damage. I don't think ALL of the various attack and defense modifiers get shown in the log.

Dexterity. Khalid has +1 ranged THAC0 from his 16 Dex, Jaheira has none from her 14 Dex.

The modified numbers are compared with thac0-based number you need to roll to succeed against the AC the opponent has.

Since Khalid has more fighter levels than Jaheira he has a lower base thac0 so he has needs a lower roll to hit an opponent.

I'm assuming Khalid is a level 4 fighter who gets a base thac0 of 17 while Jaheira is a level 3 fighter which gets a base thac0 of 18 and the opponent has a base AC of 1. So Khalid needs at least a 17-1=16 to hit AC 1 while Jaheira needs at least 18-1=17.

Actually, I believe that in the Infinity Engine games, sling bullets and arrows are a special type of damage called "missile"; in particular, this means that sling bullets are not blunt, contrary to what you would expect.

In order to do ranged blunt damage, you need to use something like Implosion (priest HLA from BG2's expansion).

(There's other quirks in the game engine; for example, the Armor of Faith provides twice as much protection against missile attacks (including arrows and sling bullets) than it does against other forms of attack, at least in the classic (non-EE) version.)