My enchant damage is listed as 765-1129. However, my max damage goes from 2.3k to 6.0k, rather than just a 1.1k increase. Why is this happening? Why is the damage I get more than the damage listed?

Fire Mastery buffs all +FireDamage to attack mods. This is one of the reasons most Chantress go with Exploding arrows.

In other words, a Chantress casts 1k Enchant with 100% FMastery. Her damage goes up by 2k. Using exploding arrow, the main arrow does 2k, then the explosion kicks in to do another 2k (so 4k to targets hit with arrow, 2k to all around). The arrow then pierces and hits monster directly behind, so that monster takes 4k, the first monster takes an additional 2k from explosion (6k total to first monster). Using ExplodingArrows, it's not hard at all to turn 1k Enchant into 6k dmg. It's not quite as effective against bosses, but you'll still get the arrowDmg + explosionDmg if you hit, or just the explosionDmg if you miss.

<edit> I did some testing with my Chantress (lvl 81), and there was no difference between my Chant damage when applied to melee or ranged attacks. There was no extra buff from FMastery other than the % it stated.
Here's the numbers I was dealing with:
1362 (max Enchant damage)
100 + 15 (FMastery was 100% + 15% from tal's peices)
2552 (pre-Chant damage)
5480 (post-Chant damage)
The buffed damage is 2928 higher, which is Chant * 2.15. This is what I expected. i.e. 2928 = 1362 + 1566 = Chant + Chant*1.15

The numbers were exactly the same if I used a bow.
</edit>

Fire Mastery.

I know it multiplies extra with melee attacks, but I can't remember the exact details. My guess is that Brevan knows as he is fond of fire sorcs. (and he has indeed posted! )

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People who live in cinder block houses can throw whatever they want.

So does the 1/3 affect for bows not apply here in HU?

And how come the enchant is buffed by firemastery for ME, but not for my merc? He only gets the 1k damage, not the 3k.

your merc hasn't mastered fire.

If I recall correctly, party members do not also get as big a buff as you recieve.

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People who live in cinder block houses can throw whatever they want.

Dang, that kinda sucks. The one single thing that a sorceress can party-wise is useless, excepting the AR.

Interesting. Quite different than vanilla. I guess here it acts as bows does there, except the character screen doesn't lie to you about it here?

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The reader should not be discouraged if...he does not have the prerequisites to read the prerequisites. P. Halmos

If i'm not wrong, the character screen lies to you. it displays the fire damage assuming you are using a melee weapon (even if you're holding a bow)

+spell damage does not apply to ranged attacks. i'm pretty sure the mechanics to these weren't modded on hell unleashed.

you can check this link from another diablo2 mod
http://modsbylaz.14.forumer.com/viewtopic.php?t=1150

Taken from the link:
+% spell damage also works with passive elemental damage sources, but only on direct melee hits.

Passive elemental damage sources include items and skills that add some sort of elemental damage to your weapon attacks, but do not allow you to attack anything with the skills themselves.
Examples: Snake Stance, Plague, Curare, Way of the Phoenix, Kraken Stance.

A melee hit occurs when you swing your weapon at something and that something takes weapon-based damage. This does not include any weapon-based missiles or splash damage (eg. Crucify spikes, Retaliate nova) that may be emitted from the target. Elemental missiles are of course considered spells.

If it was not changed in HU then melee attacks get the bonus from fire mastery twice. Once on casting and once on attack. This is what showed up in vanilla. For ranged, you only get it on casting, but it looked as though you got it twice (LCS). Brevan's tests seem to say that it only applies once here, at least as I understood it.

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The reader should not be discouraged if...he does not have the prerequisites to read the prerequisites. P. Halmos