Achieve the ACR Mastery Challenge.
I don't know 2 be exact :) But it is a good gun :)
you unlock that when you have unlocked every attachment and every camo for every gun in mw2
When you level up to level 10 you unlock kill streaks and have the option of unlocking one of the many available. From there on every 4 or 5 levels you can unlock another one.
get to level 48 and you unlock the ACR
Level 48
Rank 56 its a good gun with acog on if your into urban sniping (i do this with it) two shots to the stomach will down a guy one shot to the chest and up over all its a great gun.
get 2500 headshots
Unlock all attachments.
level 4
level 70
lvl 34