Correct me if I am wrong, but they are trained and paid to enter and quickly locate the source of the shooting and put him down.
I do a lot of gun training myself, and I'm sure the police do way more. I would feel confident entering that building if I had to (if it was my job) so I think it's safe to say those men with badges had adequate training.
When shots are ringing, they CAN find the source. it's quiet time that's dangerous to enter. When you KNOW the source of the shooting is down this hallway, you can eliminate all other directions.