Rapid Fire is unlocked when the Marksman I challenge is completed for any SMG. 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 SMGs. 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 SMGs. It increases the rate of fire by 33%.
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 a semi-automatic weapon. For assault rifles, Rapid Fire significantly increases their effectiveness at long range; but for SMGs, this attachment makes them lethal at close range but harder to use at a distance. When used with LMGs, Rapid Fire increases their deadliness at medium to long range.
Weapon level 15 (Submachine guns and Light machine guns)
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 SMGs, and by 33% on LMGs. However, as a downside, not only does the attachment increase recoil, but it now decreases range (60% for SMGs, 40% for LMGs), and increases hip spread (30% for SMGs, 20% for LMGs).