I will tell you exactly what I did and you can tell me where the problem most likely lies.

1) I used the biftool to extract all the items from the data folder into .itm form. Then I used the IEEP Item Maker to load hamm06.itm, the dwarven thrower.
2) I first changed the thaco from 3 to 5, which is the stats for Aegis-fang.
3) I went to the id screen and changed only the IDed text to Aegis-fang info, changing only the descriptive history of the weapon, the thaco and dam from +3 to +5 and erased the 9 str requirement and inserted 18 str req. Also erased the dwarves only req.
4) I then went to the 3rd icon for item maker where I changed the thaco damage modifer for both melee and ranged parts of the hammer.
5) I went to attributes screen and only changed the str req from 9 to 18/00.
6) I went to the usability screen and added human to the list (however, the first few times I might have also took off dwarf which may cause some sort of script error since the weapon was originally made for dwarf only). Then i made sure that barbarians were on the kit list of usable classes for the weapon.
7) Then I saved the new item as aegis.itm into my override folder (the saving process always had a longer delay whenever I created a version that ended up locking up the system upon importing that weapon, the one I got to work, making as little changes as possible to the weapon file saved much faster).

I think i haven't left anything out. The one I got to work finally, all i changed from the original was the adding human to the usable list, changing thac0 and damage modifiers and that's it I believe. It works fine and hits as a +5 weapon although my thac0 is only goes down 4 when the weapon is on ranged, though it does down 5 for melee. Something to do with ranged combat I assume.

Anyway, if you have any ideas what I did wrong, give me a shot. Maybe it was taking the dwarf off usability.