Call of Duty: Modern Warfare 2 adds to the custom class system by taking perks to the new level. Rather than just getting a perk at a certain level, you can now level each perk to pro level to add an ability. For example, the Ninja perk keeps you from showing up on heartbeat monitors, but once taken to pro level, your footsteps will no longer make noise, making the pro perk the equivalent of Dead Silence.
You'll also note that Juggernaut and Martyrdom are no longer perks. These are deathstreaks, which only occur after a certain number of deaths without a kill. And instead of a perk that lets you use two primary weapons, there is a perk that lets you simply change classes mid-game without dying.
Here are the Call of Duty: Modern Warfare 2 perks:
Perk 1
Marathon: Allows unlimited access to sprint. PRO: Climb over objects faster.
Sleight of Hand: Fast reloading. PRO: Aim down your sight faster.
Scavenger (lvl 13): Fully resupply from dead enemies. PRO: Extra magazines.
Bling (lvl 21): 2 primary weapon attachments. PRO: 2 secondary weapon attachments.
One Man Army (lvl 45): Swap classes without dying. PRO: Swap classes faster.
Perk 2
Stopping Power: Increased bullet damage. PRO: Increased damage to vehicles.
Lightweight: Move faster. PRO: Faster knife speed.
Hardline (lvl 9): Killstreaks require one less kill. PRO: Deathstreak require one less death.
Cold-Blooded (lvl 25): Undetectable to UAV, sentries, air support and thermal. PRO: No red name or crosshairs when targeted.
Danger Close (lvl 33): Increased explosives damage. PRO: Increased air support damage.
Perk 3
Commando: Increased melee distance. PRO: No falling damage.
Steady Aim: Increased hip-fire accuracy. PRO: Hold breath longer.
Scrambler (lvl 17): Scrambles the enemy radar in small radius. PRO: Delays explosions from enemy claymores
Ninja (lvl 29): Invisible to heartbeat sensors. PRO: Move silently.
SitRep (lvl 37): Detects enemy explosives and tactical insertions. PRO: Enemy footsteps become louder.
Last Stand (lvl 41): Stay alive for a brief time after death with pistol. PRO: Keep your equipment in Last Stand
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