AC modifier
From FOnline: 2238 Wiki
Armor Class modifier, or "AC mod" for short, is a property of ammunition that affects the AC of any target that said ammunition is fired at.
Example: If your target's AC is 100, and you're using ammunition whose AC mod is -10, your target's AC will effectively be lowered to 100 - 10 = 90 when your chance to hit that target is being calculated.
