This is technically not a cheat. It lets you choose between single shot and burst fire by simply pressing "M". Using single shot weapons is much easier with this (pistols/g3). Whether its morally ok or not is up to you.
http://callofduty.filefront.com/file/MrNickles_Fire_Rate_Toggle_Script;84606
and here is the thread discussion about it
http://www.totalgamingnetwork.com/main/showthread.php?t=140631