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Hardline

From the Call of Duty Wiki

The Hardline perk icon.
The Pro version of Hardline.

Hardline is a perk only available in Modern Warfare 2. It is a Tier Two Perk which is unlocked at Level 9, and allows the player to gain their killstreaks with one less kill. For instance, if you had UAV Recon, Care Package and Sentry Gun as your killstreaks, rather than needing 3, 4 and 5 kills respectively, you would only need 2, 3 and 4 kills. It is useful with all classes, although especially useful with classes with guns which carry little ammo, or ammo which is used up quickly, such as the Vector, because it means you need fewer bullets to get the killstreak.

Hardline Pro is unlocked after getting 40 2-kill streaks, regardless of getting a killstreak reward for it. It makes Deathstreaks activate one death earlier.

there is also a glitch using this perk and one man army to make you have 2 of the same killstreak reward in 1 life

Perks of Call of Duty: Modern Warfare 2
Tier 1 Perks Marathon · Sleight of Hand · Scavenger · Bling · One Man Army
Tier 2 Perks Stopping Power · Lightweight · Hardline · Cold-Blooded · Danger Close
Tier 3 Perks Commando · Ninja · SitRep · Steady Aim · Scrambler · Last Stand
Kill Streak Perks 3: UAV Recon · 4: Care Package · 4: Counter-UAV · 5: Sentry Gun · 5: Predator Missile · 6: Precision Airstrike · 7: Harrier Strike · 7: Attack Helicopter · 8: Emergency Airdrop · 9: Pave Low · 9: Stealth Bomber · 11: AC-130 · 11: Chopper Gunner · 15: EMP · 25: Tactical Nuke
Death Streak Perks Copycat · Painkiller · Martyrdom · Final Stand