A run of "The Pit" - the very first Special Ops Mission in Call of Duty Modern Warfare 2 will net you this, and you can go for the 3 star as done in the video, easily !
In case you can't see, I used the MP5K Holographic and the SCAR-H Foregrip - both medium fire rate but high accuracy guns for maximum aim. Note that this is the different from the pit in the S.S.D.D Mission. Here, you receive no accuracy bonuses and so you can afford a little extra spray - but it will waste time if you need to reload. Also, killing a civilian will not reduce your time but bump you down to a 2-star regardless of your time. Finally, you MUST hit every target - if you miss one, you are forced to go back.