Another way to do this is when Leon in the gun shop after the zombies killed the man. Get the shot gun from him and go to the bus that has crashed. Make sure you do not use the shot gun while going to the bus. Once inside the bus, kill the two zombies (on the floor and walking). Go to the back of the bus, where Leon entered, and aim at the screen.
In the first scenario as Claire, you can shoot the screen in the hall where the S.T.A.R.S. room is located (where you get the fax). It is not in that room, but it is in that hallway. Keep trying until you get it.
As Hunk in the green crow hall where you fight the second Tyrant, go around the second corner in that room. As you shoot at the Tyrant with a shotgun it will shoot the screen.
You can shoot the screen anywhere you can point at it with any gun except the grenade launcher, BOW gun, rocket launcher, or flamethrower.
To unlock the gattling gun, complete senario B with an A ranking without saving. This can be done simultaneously with unlocking the sub-machine gun. Also the rocket launcher obtained at the end does not decrease your rank.