Recoil

Definition
Recoil is the movement of a weapon caused by firing the weapon. It can be explained by Newton's Third Law which states "To every action there is an equal and opposite reaction." The action of the bullet accelerating forward is matched by the reaction of the weapon accelerating backward.

This movement causes the aim of subsequent shots to suffer unless the user pauses to re-aim the weapon.

In Game
Call of Duty represents recoil in three different ways:
 * 1) Visual recoil - While firing, the weapon moves slightly on the screen. This movement is bi-directional meaning, for example, if the gun sways to the right it will quickly sway back to the left if the player keeps firing. If the player stops firing, the weapon will quickly reset back to the original aim point.
 * 2) Accuracy recoil - Subsequent shots will not hit the center of the crosshairs.
 * 3) Aim recoil - While firing, the crosshairs will actually move from the original aim point. This movement is directional meaning it will never counteract itself. That is, a weapon that aim recoils up will only ever aim recoil up, never down. This type of recoil can be countered by manually adjusting the aim through the mouse on PCs or the right stick on consoles.

Some weapons, particularly sniper rifles, exhibit Sway. Sway is very similar to aim recoil but since it is not caused by firing the weapon so it is not technically recoil. Sway affects the aim of the first shot, while recoil affects the aim of all shots after the first.

Example
The Barrett has both accuracy and aim recoil. After firing, the crosshairs will move to a different aim point. This is aim recoil. If the user fires while the crosshairs are aim recoiling, the shots will not always hit the center of the crosshairs. This is accuracy recoil. Accuracy recoil is most noticeable when using an ACOG Scope with the Barrett as the ACOG reduces aim recoil but doesn't affect accuracy recoil. That is, the crosshairs will have stopped moving but firing too quickly still results in shots missing the center of the crosshairs.

The RPD as all three types of recoil, but adding the Grip removes aim recoil. With the grip and fired in automatic mode, the weapon will shake side to side as well as slightly up and down, causing a rectangular pattern.