I use to have that problem till i realised i wasn't the kind of person who can hop on and be on the mark first match.So for about 30-45 minutes i play against veteran bots in a TDM a few times with the honey badger red dot and extended mags or with foregrip and extended mags.A good way to hone your aim in is also run double the sensitivity you normally play at while doing this.Then when you go into multiplayer put it back.You will find that your aiming is much smoother than before.Most people don't realise but running a high sensitivity does increase your reaction time abit, since in order to run at very high sensitivity's you need a good reaction time to correct your aim.I run at 20 so i can't do that.Though i am sure you get the point.
Sounds silly sure, but trying never hurts haha.Though i can almost guarantee that fighting the bots on veteran will in fact make your reaction timing much faster and aiming sharper than it would had you not.For those that don't know you can make a custom match against any kind of bots and game mode in Private match.This will surely help you out immensely.