Perks

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Perks are Call of Duty 4: Modern Warfare 's and Call of Duty: World at War 's multiplayer modifiers that passively affect game play, allowing further customization of classes. They affect available equipment, effectiveness of the player's equipment, the usefulness of the enemy's equipment, and the physical properties of the player's character. Every perk has an opposition perk which is a perk that cancels out the other. Kind of like defender and attacker.Stopping Power is cancelled out by juggernaut. Flak Jacket blocks fireworks.

List of perks in Call of Duty 4: Modern Warfare
All perks are available initially unless otherwise noted.

First Tier
Most first tier perks involve the items players spawn with or how much they spawn with.

Bomb Squad
 * Detect enemy explosives. (Unlocked at level 14)

C4 x2
 * Two blocks of Composition-4. (Available at level 1)

Claymore x2
 * Two Claymore mines. (Unlocked at level 23)

RPG-7 x2
 * Two RPG-7s. (Available at level 1)

Special Grenades x3
 * Three Flash Grenades or three Stun Grenades (note: does not work on Smoke Grenades)(Available at level 1)

Bandolier
 * Extra ammunition. (Unlocked at level 32)

Frag x3
 * Three Fragmentation Grenades. (Unlocked at level 41)

The following perks cannot be directly selected. Adding the corresponding attachment to your Primary Weapon will disallow the selection of any other Perk 1, and so has the effect of being a Perk 1.

M203 Grenade Launcher/ GP-25 Grenade Launcher
 * Attaches a high-velocity impact grenade launcher to an assault rifle.

Foregrip
 * Decreases the recoil of light machine guns and shotguns.

Second Tier
Second tier perks are generally perks that change the way players or their weapons operate.

Juggernaut
 * Increases health.

Sleight of Hand
 * Faster reload. (Unlocked at level 20)

Sonic Boom
 * Increased explosive damage.

Stopping Power
 * Increased bullet damage.

Double Tap
 * Increases rate of fire. (Unlocked at level 29)

UAV Jammer
 * Makes the player unseen to enemy UAVs. (Unlocked at level 11)

Overkill
 * Carry two primary weapons. (Unlocked at level 38)

Third Tier
Third tier perks are modifiers that change the physiques of players or what they do before and after dying (with the exception of Deep Impact).

Deep Impact
 * Increased bullet penetration.

Extreme Conditioning
 * Sprint for longer distances.

Last Stand
 * Pull out your pistol before dying. (Unlocked at level 8)

Martyrdom
 * Drop a live grenade when killed. (Unlocked at level 17)

Steady Aim
 * Increases hip-fire accuracy.

Dead Silence
 * Reduces the footstep noise. (Unlocked at level 44)

Iron Lungs
 * More steadying time for sniping. (Unlocked at level 26)

Eavesdrop
 * Hear enemy voice chat. (Unlocked at level 35)

First Tier
Special Grenades x3
 * Gives the player 3 special grenades excluding smoke grenades.

Satchel Charge x2
 * Similar to the C4 in Call of Duty 4.

M9A1 Bazooka x2
 * Similar to the RPG in Call of Duty 4, perhaps more accurate.

Bomb Squad
 * Same as it was in Call of Duty 4, you can see where enemy Satchels and Bouncing Betties are located.

Bouncing Betty x2
 * Similar to the Claymore in Call of Duty 4, mines that are planted in the ground.

Bandolier
 * Increases initial ammo for your selected weapons.

Primary Grenades x2
 * 2 frag grenades, sticky grenades, or Molotov Cocktails.

M2 Flamethrower
 * The player is equipped with a flamethrower.

Second Tier
Stopping Power
 * Weapon damage is increased by 40%.

Fireworks
 * Explosive damage is increased.

Flak Jacket
 * Explosive damage dealt to the player is decreased.

Gas Mask
 * The effects of Tabun gas are decreased.

Juggernaut
 * Damage from bullets is decreased.

Camouflage
 * The player will not be seen on the map even after a recon plane, however shooting a weapon without a suppressor will show them. Identical to UAV Jammer.

Sleight of Hand
 * Reloading is quicker.

Shades
 * Reduces the effect of signal flares on the player's vision. Not available on the Wii version.

Double Tap
 * Increases the player's weapons' rate of fire.

Overkill
 * Allows the player to wield 2 primary weapons (meaning no pistols). Same as Call of Duty 4.

Third Tier
Deep Impact
 * Gives the user's weapon more penetration power.

Extreme Conditioning
 * Gives the player the ability to sprint longer.

Steady Aim
 * More accuracy when firing from the hip. Note: this does not help with aiming down the sights.

Toss Back
 * When tossing back a grenade, it resets the timer.

Second Chance
 * Almost the same thing as Last Stand in Call of Duty 4 except you survive much longer and another player will be able to revive them.

Martyrdom
 * When the player is killed, they drop a live grenade.

Fireproof
 * Damage from fire done to the player is reduced.

Dead Silence
 * The player barely makes, if any, noise when walking or sprinting.

Iron Lungs
 * Gives the user the ability to hold their breath with a sniper rifle for 10 seconds instead of 4.

Reconnaissance
 * Gives the player the ability to see all artillery, dogs, and tanks on the map.

Vehicle Perks
Water Cooler
 * Decreased turret overheating.

Greased Bearings
 * Increased turret rotation speed.

Leadfoot
 * Increases the speed of the tank.

Ordnance Training
 * Boosts the firing rate of the tank's main firing cannon.

Coaxial Machine Gun
 * Allows the driver to use a coaxial machine gun only in the front of the tank.

(Please note that Shades, Iron Lungs, Reconnaissance and Vehicle perks are not available on the Wii version of Call of Duty: World at War.)

List of Perks in Modern Warfare 2
Stopping Power Martyrdom
 * Increases bullet Damage (This Perk was seen in the Modern Warfare 2 teaser trailer)
 * Drop a live grenade when killed (Robert Bowling said they were working on the perk)