Before you brush this off and say, "Oh, I can do that already," think twice. I have only played a handful of opponents who could shoot through the grates in the walls effectively. Most players can make the shots 5/10 times; good players make them 7/10 times; great players make them 8-9/10 times. In order to use the grates to your greatest advantage, you must know all of the angles you can shoot from in order to hit your opponent.
 
It is close to impossible to accurately describe where each shot and angle is, as there are so many different ones. So instead of taking screenshots and describing them, here's a demo of all of them. See demo.

Next: Shooting the RA Hole Button