Here are a few areas I know of, where you can kill people with a Sniper Rifle/Bazooka, within 5 seconds of the game. Mission 1 (Firefight): Run straight for about 7 seconds. Look straight ahead, there should be a guy walking around. Level where you plant charges on Pylons (Mission): Take out your bazooka, turned around, stand next to the rock behind you. Look straight at the bridge, and fire your bazooka. You'll get up to 3 kills according to where you aim. If lucky, you can get Enemie Armor.
That's all I know of currently, I will add more later. Spots found by The Ironhide.