Juggernaut is a second tier Perk in Call of Duty 4: Modern Warfare and Call of Duty: World at War. In Modern Warfare, it decreases bullet and explosive damage the player takes by 25%. It cancels out and is canceled out by the Stopping Power and Sonic Boom perks. In World at War it decreases bullet damage by 25% and does not have any effect against the Fireworks perk. The Flak Jacket perk was created to be the antithesis to Fireworks, making Juggernaut less powerful in World at War. In almost all situations, Juggernaut will allow the user to survive one more shot. A Juggernaut symbol (a plus sign in a red and white circle) will appear just below the crosshairs when a player shoots an enemy player with the Juggernaut perk
Call Of Duty 4 : Modern Warfare
Juggernaut is exactly the same as Stopping Power but in reverse, making it take one more bullet to die instead of one less bullet to kill. Weapons that are extremely dependent on Stopping Power such as the M21, M40A3, M16, and M14 are much worse at dealing with Juggernaut users.
In Call of Duty 4, Juggernaut has some advantages over Stopping Power. This is because Juggernaut not only cancels out bullet damage, but it also affects explosive damage and falling damage. So if a Juggernaut user and a Stopping Power user engage in a firefight, the Juggernaut will cancel out Stopping Power's bullet damage boost (and both will deal the same bullet damage to each other), but the Juggernaut will still take less damage from explosives. However, Stopping Power is more effective on most weapons because it can be used against multiple enemies in under 5 seconds, whereas Juggernaut essentially disappears after the first hit, and health regeneration takes 5 seconds
Call Of Duty 5 : World At War
Juggernaut is essentially the same in World at War as it was in Modern Warfare. However, with the inclusion of Flak Jacket, explosive damage is not reduced by Juggernaut anymore.
The Perk-a-cola in Nazi Zombies Juggernaut is also used in 3 of the 4 Nazi Zombies levels under the name of 'Juggernog'. It allows the player to take 3-6 hits from a zombie before being downed and costs 2500 points. In 2 of the 3 levels (Der Riese and Zombie Verrückt) it requires the power to be activated before the player is able to acquire it.