Bandolier

Bandolier is a Tier One Perk in Call of Duty 4: Modern Warfare and Call of Duty: World at War whilst in Modern Warfare 2 it is replaced by a new perk, Scavenger Pro.

Call of Duty 4: Modern Warfare
Bandolier is unlocked at Level 32. Bandolier is not advisable to use when playing Search and Destroy as there are usually only six enemies and you are given one life; the need for extra ammunition becomes less important. This perk can be helpful when using a Shotgun and the perk Sleight of Hand in such levels such as Vacant, or even Shipment, where ammunition and fast reloads are important (especially on Shipment, since if you get a Helicopter it can be devastating). This perk works well with weapons with a high rate of fire, or players who have a tendency to use up ammunition quickly in general. A good combination would be to use this perk with Double Tap and Steady Aim, for faster fire, better hip-fire accuracy, and plenty of ammunition, in case you use more of it than you realize. It is also a good perk for snipers, due to the limited amount of reserve ammunition provided from spawning. If you are going to snipe the whole match, and you run out of ammunition, the only other gun left from the loadout is a pistol, unless you use the Overkill perk, so the added ammunition reserves for the sniper rifle could keep you alive longer, and prevent the worry over the ammunition left after each shot.

Call of Duty: World at War
This perk is unlocked at level 40. It will give any weapons in the loadout it is used with the maximum amount of reserve ammunition you can carry. It's best used with high Rate of Fire weapons that tend to use up the magazine faster than they do damage to the target. A good example is the PPSh-41. The gun has an extremely high RoF, meaning it would use most of the magazine before doing any effective damage, so more ammunition is needed than normal. Bandolier can remedy this. It is also good for weapons that have a low ammo count to begin with, such as the Thompson. Combine this with the Extended Magazine attachment (The Round Drum to be more specific) for this item, and it will maximize the weapon to a good potential.

Bandolier is also good for long ended matches, such as "Hardcore Team Deathmatch". If you are careful and maneuver yourself efficiently, then you can last for a long life, and if you don't have this perk activated, then you might find yourself out of ammo. Keep that in mind because in Hardcore, your HUD is removed, which means no ammo count. Bandolier eliminates the worry over whether ammunition is running low.

Bandolier is good for any SMG with an Extended Magazine, and any MG, especially the MG42 and FG42, seeing as those guns consume ammunition very quickly.

Call of Duty: Modern Warfare 2
This perk has been replaced by the new perk Scavenger Pro, which allows the player to pick up ammo and equipment from dead enemies. Scavenger Pro is bandolier as you get more ammo, usually x3 plus the advantage of normal scavenger.

Trivia

 * In real life, a bandolier is a long belt slung over the shoulder(s) and across the chest, that is used to carry large amounts of ammunition. In the case of machine guns, a bandolier can also be made up of linked ammunition belts wrapped around the user, without needing a separate garment to carry them.