In Call of Duty: World at War, it is unlocked at rank one in the "Heavy Gunner" class.
Flak Jacket decreases all explosive damage taken by 25%, and caps the maximum damage taken from any one explosive (excluding a direct hit from a grenade launcher, Bazooka or a tank shell) to 75. In effect, this allows the player to survive any single indirect explosive attack. Players who find themselves constantly killed by Grenades, Bouncing Betties, Tanks, Satchels, Artillery Strikes or M9A1 Bazooka will find this perk to be very useful. This perk has very little effect in Hardcore, due to decreasing HP of players, since the 75 damage cap is far above the maximum health level. Getting stuck with a Sticky Grenade with the Flak Jacket perk equipped will still always result in a death.
Flak Jacket reduces all incoming explosive damage to 35% of its original value (9% in Hardcore game modes), a decrease of 65%, substantially more than in World at War. Flak Jacket Pro reduces all fire damage to 5% of its original damage, making the Flamethrower and Napalm Strike nearly useless against the player. As an added effect, any thrown grenade picked up by the player will have its fuse reset to 2.5 seconds, allowing safe grenade returns and therefore fulfilling the same role of Toss Back from Call of Duty: World at Warand Fast Hands from Call of Duty: Black Ops II.
When the Flak Jacket is equipped it will give the character model extra heavy armor pads on his body resembling that of a Juggernaut. This will count towards all factions besides colors, faces and small details like a helmet. However, it should be noted that Flak Jacket does not make a player a larger target or any slower.
Flak Jacket returns as a Tier 1 perk in Call of Duty: Black Ops II and can be found by using the Access Kit on certain levels in the campaign such as "Pyrrhic Victory" and "Suffer With Me". The effects of Flak Jacket are identical to its Black Ops counterpart, which makes explosives do less damage to the player. All explosives do 35% of their original damage to people with the Flak Jacket perk (20% in Hardcore), though it does not help against the user's own explosives, including cars triggered to explode.
"Reduce damage taken from explosives. No slowdown from explosive damage. Reset grenades time on throwback."
— In-game description.
Flak Jacket returns in Call of Duty: Advanced Warfare, with similar effects to the Black Ops rendition. Players using this perk will take less damage from explosives, and will not be slowed down when receiving explosive damage. Also, when throwing back a grenade, the grenade timer resets.
This perk is especially useful when attempting to destroy enemy scorestreaks as many of them use explosive weapons.
An update was added that allowed users to not flinch when being hit by grenades such as frag or semtex grenades.
The Demolished portion of the Flak Jacket Pro challenge is not featured in Combat Training due to there only being Team Deathmatch and Free-For-All gametypes.
The throwback challenge cannot be obtained by throwing one's own grenades on the ground, and then throwing them to a safe distance away. However, it can be obtained by picking up an enemy frag just as it is blowing up, even if the player would not have the time to throw it away, thus dying from the explosion.
If the player with Flak Jacket is near a teammate that got stuck by a Semtex or a explosive tip arrow and it blows up, the player will still die even though they have Flak Jacket on.
The Arctic Spetsnaz model with Flak Jacket is wearing spiked shoes. It is the only model to have this feature.
All Flak Jacket models have a patch of the flag of their country on their left arm.
Oddly, the Tropas's model has a USSR's logo patch writing on their shoulder, similar to the one Spetsnaz have with Flak Jackets.
The Flak Jacket perk also reduces damage done by the Chopper Gunner and Gunship as they count as explosive rounds. This only appears if there is no direct impact.
The character model for the Chopper Gunner Minigun operator will always take the model of a soldier with Flak Jacket equipped, regardless of faction or Tier 1 Perk equipped by the ground operator.
The model in the perks picture is an Op 40 with Flak Jacket.
The player cannot get the "Protected" challenge by surviving their own explosions.
The Arctic Black Ops and Spetnaz Flak Jacket model was originally going to wear a balaclava, but it was cut in the final version.