Rapid Fire is unlocked when the Marksman I challenge is completed for any submachine gun. Obtaining 30 kills with the Rapid Fire attachment will unlock the Akimbo attachment. It increases rate of fire by around 25%. In addition to indirectly increasing recoil by giving the weapon less time to recenter between shots, it also reduces centerspeed.
Rapid Fire is available for all submachine guns. It is the same as its Call of Duty: Modern Warfare 2 counterpart. It is the only way that base damage-per-second can be increased in any way in Call of Duty: Black Ops due to the removal of the Stopping Power perk, and is therefore very popular on most submachine guns. It increases the rate of fire by 33%.
Rapid Fire is nearly identical in performance to previous installments. However, the increase in rate of fire is 25% (like in Call of Duty: Modern Warfare 2), as opposed to the 33% increase in Call of Duty: Black Ops. It's unlocked at Weapon Level 11 for the submachine guns and is also available for the light machine guns at Weapon Level 14. Two burst-fire assault rifles unlock Rapid Fire at Weapon Level 12: the M16A4 and Type 95, and one semi-automatic, the MK14.
On the M16A4 and Type 95, Rapid Fire increases how fast the burst is fired, which means that bullets have tighter grouping. However, the delay between bursts remains unchanged at 0.2 seconds. Rapid Fire on the MK14 is unique; instead of increasing rate of fire, it increases the MK14's firecap significantly, since it is semi-automatic. For assault rifles, Rapid Fire significantly increases their effectiveness at long range; but for submachine guns, this attachment makes them lethal at close range but harder to use at a distance. When used with light machine guns, Rapid Fire increases their deadliness at medium to long range.
Completing "Karma" (SP) Weapon level 15 (SMGs and LMGs) (MP)
The Rapid Fire attachment returns to Call of Duty: Black Ops II. The attachment is only available to submachine guns and light machine guns. It increases the rate of fire by 51% on submachine guns, and by 33% on light machine guns. However, as a downside, not only does the attachment increase recoil, but it now decreases range (60% for submachine guns, 40% for light machine guns), and increases hip fire spread (30% for submachine guns, 20% for light machine guns).
Rapid Fire reappears in Call of Duty: Ghosts, available once again to submachine guns and light machine guns. It increases the fire rate on compatible weapons by ~17%, which is a lower amount than in previous games. Unlike its Black Ops II counterpart, where rate of fire was greatly increased at the cost of reduced range and increased hipfire spread, Rapid Fire in Ghosts does not reduce range or decrease hipfire accuracy but it does increase recoil by 5%. In Extinction mode, it is possible to mount the Rapid Fire attachment on assault rifles as well (except the SC-2010, due to it being semi-automatic in this game mode).
In addition to submachine guns, Rapid Fire in Exo Survival mode can be mounted on two assault rifles as well (Bal-27 and HBRa3), but as a downside it is unavailable to heavy weapons (except the Ameli, which can still have it).
Note: Rapid Fire doesn't increase the first fired rounds of the ASM1, SN6, Bal-27 and HBRa3.