The M24 is a sniper rifle and the IA2 AR is a machine gun.
I found this easiest to do in Deathmatch.
NOTE this can be unlocked on any map as long as you have the DLC.
My strategy: I started off with the IA2 AR and I tried killing 3-4 of the enemy players with his gun within the first few minutes of the game, and then I switched to the M24 and I would spend the rest of my time attempting to sniper a player with this gun. This gun is a 2 shot kill, or 1 shot head shot. It's a little slow, so I'd suggest turning up your sensitivity to make it a lot easier.
It's easy enough to obtain legitimately and even easier in a boosting party.