Stopping Power

Stopping Power is a Tier Two perk in Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2 that increases the amount of damage of a user's primary or secondary weapons by 40%. In almost all situations, this means that one less bullet will be required for a kill. Because of this fact, Stopping Power is significantly more useful on weapons with low rates of fire, where one bullet can mean a lot. Inversely, it is relatively ineffective on weapons with high rates of fire because for these guns one bullet does not mean as much. The effects of this perk are canceled out by the Juggernaut perk in Call Of Duty 4 and Call Of Duty: WaW, but in Modern Wafare 2, there is no Juggernaut so it is canceled out by the Painkiller death streak 'reward'. Stopping power will not increase the damage of explosives, such as C4, Satchel Charges, Bouncing Betties or Claymore Mines; as this is accomplished with the Sonic Boom, Fireworks or Danger Close perk. It also has almost no effect in Hardcore mode since most guns are one-shot-kills meaning it can be switched out by another tier 2 perk.

Call of Duty 4: Modern Warfare
In Call of Duty 4: Modern Warfare, Stopping power is most often used on the M16 and most sniper rifles, as it usually deals a one-hit-kills. It is also the most commonly used perk in the game. However, it is not advisable for use with shotguns, as they are already very powerful without it. Stopping Power is also more ammunition-efficient, as it decreases the number of shots required to kill a target (although this effect is less noticeable). One of the most overlooked guns, the M16, (due to it being the standard issue weapon), can also be one of the most effective. An M16 with a Red Dot Sight, Bandolier, and Stopping Power is a very deadly combo; as the M16 fires in three-round bursts, a simple grouped shot on a target will instantly kill it, and 1-2 shots will kill anyone with Juggernaut. Testing shows using this combination can massively increase your kill count, (but deaths are still based on your situational awareness), meaning for anybody who plays in the 'Assassin' style can use this in CQB to rack up points when out of their comfort zone. But beware, having a high kill count, high points, or a good K/D spread (in a particular game) will mark you as one of the primary targets for a helicopter.

Call of Duty: World at War
Stopping Power is used in the same way as in Call of Duty 4. Players will often use them with weapons that have a high rate of fire, like the Thompson or the PPSh-41, which is actually inadvisable, because Stopping Power is much more effective on weapons with lower rates of fire. If you aren't worried about ammo-efficiency, Double Tap is slightly better for these guns or any gun with a damage of less than 30.

Stopping Power is very effective when combined with rifles and bolt action rifles. When used on any bolt action rifle, one shot can incapacitate an opponent if the shot hits at the stomach or above, with or without a Sniper Scope.

The downside to the perk is that any effects of it are countered well by Juggernaut, and people with damage perks can't have a defensive or support perk, so they are easy targets if caught off-guard.

Call of Duty: Modern Warfare 2
Stopping Power made its return again in Call of Duty: Modern Warfare 2. It belongs to the second tier and is unlocked from Level 1 online. It has an identical picture to that of the Call Of Duty 4 Stopping Power picture.

The Pro version is unlocked after you have got 250 kills with a class using Stopping Power. The Pro version allows the user to have increased damage against enemy vehicles such as Harriers and Pave Lows

Once again, Stopping Power is most useful when used with single shot weapons, such as the Intervention. With no Juggernaut to counter it in Modern Warfare 2, Stopping Power is only hampered by someone using the Painkiller death streak. It has no extra effect on a riot shield.

It is notable that Stopping Power has minimal effects on the FN FAL Battle Rifle, in that the Rifle will take 2 shots at medium range and 3 shots at distant range to drop a target, regardless of if Stopping Power is on or not. This is likely due to the FAL's initial damage being 50/35, and Stopping Power only provides a 40% bonus - Stopping Power boosts the long-range damage only to 49, which will still need 3 hits to kill a 100 HP target.

Trivia

 * The name of this perk comes from the real-life colloquial term "stopping power", used to describe the ability of ammunition to sufficiently incapacitate its target. In general, caliber correlates directly with stopping power, though a round's design is also a factor (as is the case with hollow point or explosive-tipped ammunition).
 * Stopping Power seems to be the most used perk in all 3 games.
 * The perk picture was seen in the Teaser trailer in early 2009.
 * Stopping Power can commonly be seen in use by players who hipfire excessively or have a lack of accuracy.