Critical Hits

Fallout's critical hit system removes the chance-based critical hit trigger for characters, replacing it with a critical meter which builds up with each successful V.A.T.S. hit and, when filled, can then be triggered by the player just before any V.A.T.S. attack, guaranteeing a hit with extra damage. To fully fill the critical hit meter it takes 20 hits in V.A.T.S . minus your Luck Score. Ex. If your Luck Score is 10 then it will only require 10 hits in V.A.T.S. to fill your critical meter.

Outside of V.A.T.S. critical hits cannot occur unless something increases your critical hit chance, such as with Overdrive, or if the attack is a Sneak Attack.  The number of critical strikes will always equal the number of times a Critical was activated in V.A.T.S. plus the number of sneak attacks.

Note that although the system counts a sneak attack as a critical hit, a sneak attack does not count as a critical hit for purposes of critical hit damage bonuses.

Sneak Attack

Sneak attacks are attacks that hit a target that is unaware of the attacker. When a sneak attack hits a target it follows the same damage rules as a Critical Hit. There are several Perks that can increase this damage to make them more effective. If you have one of these perks, the perk happens after you maximize damage. If you have the Ninja Perk the damage is maximized and the multiplier replaces the normal 1.5x multiplier.

Ex. the first rank of Ninja grants a 4x multiplier to melee sneak attacks so it would replace the 1.5 in the formula making it.
Melee Sneak Damage = Maximum Damage * 4 + Damage mods  

Ex. for ranged attack for rank 1 in Ninja perk
Ranged Sneak Damage  = Maximum Damage * 2 + Damage mods

Sneak Attack Weapons

To be able to use a sneak attack you must first have a proper weapon to do so. A gun must have some form of silencer attachment to use the sneak attack. A melee weapon can only sneak attack if its weapon speed is Fast or Very Fast. 

NOTE: Once you make a sneak attack a Contested Skill Check is made. Their detection vs your Sneak. This only occurs if you are out of line of sight of any creature the witnessed the effects of the attack. 

Critical Damage

Weapons have a predefined critical damage which is added when a critical hit is executed, usually equal to the displayed weapon damage (including weapon mods). In Fallout there are several ways to alter the critical hit damage. One way is through modifying a gun by adding, e.g., a Calibrated receiver or Photon exciter. These two mods add to the critical hit damage. For some weapons there are receiver mods that will increase the critical damage even further, such as the Photon agitator.

Critical damage can also be increased in small increments by reading perk magazines and collecting bobbleheads.

Critical Damage Formula

Ranged Combat

Damage Critical = Maximum Damage + Damage mods * Critical Multiplier

Melee/Unarmed Combat

Damage Critical =  Maximum Damage * 1.5 + Damage mods * Critical Multiplier

Crippling Hits

Whenever you roll 1-5 on an attack roll, you cripple the target. Roll a d6.
On a 1: Cripple the Head
On a 2: Cripple the Primary Arm
On a 3: Cripple the Off hand Arm
On a 4: Cripple the Torso
On a 5: Cripple the Right Leg
On a 6: Cripple the Left Leg.

Weapon Condition

If you roll a 96-100 and miss, a 100 is always a miss no matter your skill point level, your weapon gains one faulty condition. Once a weapon gains ten faulty conditions it breaks and becomes unusable. To repair any amount of the weapon you must do so using its full repair cost with a successful repair skill check.