I was curious so I went over to mudbytes and pulled the Rom23 code. I've made all the numbers positive = good and negative = bad for ease of reading. The actual code is the reverse because DnD 2? I'm simplifying the math a little, but not much.

Basically there is a linear equation that puts the "chance" to hit for a character based on level somewhere between -18 and 26 (with no hitroll). So a level 1 character has ~-18 base "hitroll" and level 51 has 26 "base" hitroll. Hitroll itself is (more or less) just a static bonus on top of this. Basically, 20 hitroll at level 1 gives you a "2" chance to hit and 46 at level 51.

This number by itself is meaningless, as it is compared against AC to determine the actual hit. AC isn't the AC you see in score, it is that number divided by 10. So -100 AC is a "10" in chance to avoid.

The 2 numbers are compared, and if AC is greater than "hit chance" by more than 20, then you get 1/20 chance to hit (20 on a random roll is auto hit).

If AC is less that hit chance by more than 20, you get 19/20 chance to hit (1 on random roll is auto miss... I'm not sure this is reality in CF).

If the 2 numbers are within 20 you get a sliding scale.

As an example, A level 15 warrior has a base "hit chance" of -5. If you had 10 hitroll, his adjusted "hit chance" is 5.

AC of -100 gives us a "10" avoid chance. So 10 - 5 = 5. We'll get hit about 3/4 ths of the time.

The problem with "gearing for AC is that everything above "15" (or AC -150 or higher) is penalized by being put into a different linear equation which is much much flatter in slope. -200 AC is required to get a "16" avoid chance. -250 for "17" and so on. After -150 AC, instead of getting +1 avoid chance for every -10 AC (as seen in score) you need -50 AC to get that +1. To avoid damage entirely (or at least get hit only 1/20th of the time) you have to beat the hit chance with AC by 20.

So -150 AC at level 15 vs. an NPC with 0 hitroll is basically all misses (base chance -5), but as the levels scale up, AC sucks harder and harder.

If we look at a level 51 warrior with 20 hitroll (warcry + berserk + stats would easily get us *at least* this), the amount of AC needed to get anything higher than 1/20th chance of miss (which is always there) would be -2000 or "52" avoid chance. That only gives you an extra 5% chance of of a miss. Your 52 vs. their 51. To get back to the 5% chance of getting hit we had at level 15, you need -3000 AC ("72" avoid chance). That is for as low of hitroll as you are likely to see. Even if hitroll from gear,skills, and stats didn't exist, you'd still need -1750 AC to have a 5% hit rate.

For NPC's with 0 hitroll (which they don't have to have, depends on area writer) the maximum level it can be and still have a decent shot at getting the 5% hitrate is basically level 15 ("-5" hit chance). By level 20 they get "0" hit chance with 0 hitroll which means you'd need -400 AC to get 5% hit rate.

How much of this applies directly to CF is hard to tell, but my best guess is that this code exists almost exactly as it did in Rom2.3

And certainly consistent with all my observations. Obviously I can't test the precise details but the principles I've found are:

A: ac is huge at low level because anyone who has hit roll to hit you doesn't have damroll to kill you. A sound understanding of this is why magi should dominate low ranks. But few gear appropriately so they don't.

B: beyond about lvl 20 (enemies, not you) you can no longer use ac as your defense. Probably because you are now getting the diminishing returns.

I think this makes for interesting play because gearing strategy needs to be different at different ranks.