Stun Grenade

The Stun grenade is a grenade in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2. When it explodes, it temporarily blinds anyone in its blast radius, greatly slows victims down and does a small amount of damage. 

Call of Duty 4: Modern Warfare
The stun grenade is one of three available special grenades. One is normally given, but the player's inventory of special grenades can be increased to 3 if the perk Special Grenades x3 is used. They are normally used tactically by players, and a list of viable tactics can be found below.

Call of Duty: Modern Warfare 2
In Call of Duty: Modern Warfare 2, two stun grenades are now given instead of one. Wearing a Blast Shield negates the slowing and blinding effect of stun grenades, giving Blast Shield wearers a significant advantage in indoors fighting. The same benefit is received from a Riot Shield.

The Stun Grenade in Modern Warfare 2 is completely unchanged from Call of Duty 4, so the tactics are generally the same.

Tactics
The stun grenade is most often used around a corner or thrown through windows, where the corner or window sill will prevent the thrower from being stunned. It is typically used in close quarters combat to allow the player to kill dazed enemies easily. Affected targets move and turn considerably slower than normal making it hard for them to aim or evade fire. There is an initial flash but vision is quickly returned. If within the radius, the player throwing it will be affected by the grenade, but not his teammates (unless friendly fire is turned on). The effect radius is smaller than the Flash grenade but makes all targets in the radius extremely vulnerable. Players with 3 may throw them all at the single location, allowing them to travel a moderate distance before the stun effects wear off. Some players use this weapon because it deals a small amount of damage to its target, thus indicating to the thrower whether or not a target has been stunned. Stun grenades can also be used to slow enemies at the beginning of a Search and Destroy round. Because enemies will not be able to rush or take positions as quickly, it can put them at a significant disadvantage in certain situations. A common tactic used to get kills with explosives is to throw a stun grenade into a crowded area and then fire or throw an explosive into the stunned area. Because the enemies move slower, they are easier targets and can't run from uncooked frags/semtex as easily. Additionally, killing stunned enemies using explosives does not expose the thrower to direct enemy fire if the enemy happens to be pointing in the right direction. This is great for slowing the target and knifing him.

A direct impact from a Stun or Flash grenade, or even just the stun/flash itself, will always kill somebody that is in Last Stand. Also, in Hardcore game modes like Hardcore Search and Destroy and Hardcore Team Deathmatch, a direct Impact hit from a stun grenade is enough to one-shot anyone at full health unless they're using Juggernaut. Although it is extremely rare, stun or flash grenades explosions can kill a heavily wounded target, as it does 1 damage. If the player cooks a stun grenade and goes into Last Stand, it will turn into a frag grenade. Stun Grenades will not go off in mid-air if thrown straight up like flash grenades would; rather they will detonate after hitting a surface.

When a player is stunned by an enemy, a sometimes viable tactic is to throw a stun grenade back at the enemy. Because the stun time is dictated by the distance from the center of the blast, if the enemy is closer to the new blast, they will be stunned longer than the player, giving the player a chance to recover and counter attack.

Additional Tactics in Modern Warfare 2
Equip Stun grenades on a riot shield class with a throwing knife and when the player sees an enemy, throw the stun grenades, run up to the enemy, and finish them with the throwing knife.

Stun grenades, along with flash grenades, can be used to temporarily disable enemy Claymores in the blast radius. It is a good idea to throw one into a building/area which is likely to have claymores planted, as it will then be safe for the player to enter. If the claymore has been affected by the grenade, it will start sparking as an indicator.

More than that, stun grenades explosion will detonate fuel barrels, which can be used to kill enemy.

Trivia

 * On the Wii version of Call of Duty 4, players are still able to turn at their normal speed, making stun grenades much less useful.
 * It is possible to kill someone with the flash of the stun, as it delivers 1 damage.
 * The icon on the Create-A-Class menu shows the stun grenade as being red in the middle, while in game, the two stun grenades positioned next to the Equipment will have blue in the same area.
 * On the Multiplayer map "Highrise," if a Stun grenade is thrown off the edge, and it detonates, it will produce an explosion-like sound, unlike the Flashbang.
 * The throwing animation is much quicker than the flashbang animation, making it more useful when surprised by an enemy.