Which one?
Gold: unlock all attachments
Grey/Silver: 500 kills (need to be prestige)
Grey/Silver with headshot icon: 250 headshots
White Skulls: 2500 kills
Gold Skulls: 1000 headshots
check here for all the titles and emblems: http://callofduty.wikia.com/wiki/Callsigns
You have to get 10000 kills with the intervention.
Chat with our AI personalities
when you are level 4 you unlock the intervention and 50 cal.
Well, as you all know the 'Ta-Da!' title is VERY rare. I know how to unlock it! What you have to do is you have to kill 25 players within 5 seconds after respawning on a Tactical Insertion. That's how you unlock the 'Ta-Da!' title!
you unlock it by getting 50 kills with the stealth bomber. this is the Spirit 1 challenge.
Achieve 1000 headshots with the ACR. You must be prestiged at least once to unlock this challenge.
get to level 48 and you unlock the ACR