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Critical Chance

This is a modifier found on Weapons and Armor, as well as being a derived Attribute, visible on your character sheet. Note that in Hellgate: London, derived attributes are derived from skills and items, not your main attributes.

Your Character's Critical Chance

Your Critical Chance is a percent chance to score a Critical Hit. All characters start with a 1% Critical Chance. Use skills or items to boost it beyond this value.

Your Weapon's Critical Chance

Depending on the base Weapon statistics, a weapon may add anywhere from zero percentile points to five or more percentile points. This value is added, not applied as a percentage. So, for example, if you have a weapon that has a 3% chance of a critical hit, your total chance to score a critical hit is now 4% (3 percentile points plus the default 1 percentile point). This is further modified by Skills, item Modifiers, and Mods.

Increased Critical Chance from Mods and Modifiers

Mods that you've inserted into your weapon and item Modifiers that increase your critical chance are treated as percentile points, and are, again, added to the total. The exception is mods/modifiers that increase your critical chance against specific castes, such as "increased critical chance against beasts (+60%)", which is applied as a percentage. So, if your critical chance is currently 2%, then your critical chance against beasts (only) would be 3.2% (1.60 * 2 = 3.2%). Of course, this is assuming that the mod affects the final value, rather than being applied earlier in the calculation. The exact formula used is unknown.

Increased Critical Chance from Skills

You can get either passive or active bonuses to your critical chance from skills. For example, the Marksman skills Tactical Stance and Dead Eye increase your chance to score critical hits while toggled and passively, respectively. Once again, these values are added to the sum total, rather than multiplied.