Rapid Fire is an attachment exclusive to submachine guns in Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops, that increases the rate of fire of the gun it is attached to. It is the replacement for Double Tap and has exactly the same effect; the delay between shots is decreased by one-fourth, effectively increasing the weapon's rate of fire by one-third. Obtaining 30 kills with the Rapid Fire attachment will unlock the Akimbo attachment. Logically, this attachment adds recoil to the gun when fired in full auto mode. It should be noted that the game weapon stats are wrong. The rate of fire never caps, so there should be no worries in attaching this to a weapon such as the Vector.
Many players overlook Rapid Fire because it increases recoil and consumes ammunition at a much faster rate. Not to mention that SMGs have some of the fastest fire rates anyway. However, it can be used to increase damage-per-second or the increased recoil can be used for easier headshots, in some cases. It is the only way that base damage-per-second can be increased in any way in Call of Duty: Black Ops thanks to the removal of theStopping Power perk.