Trapped!

"In this room, there is a vicious dog, trained to kill. Next to you is a sniper rifle. In 15 seconds from when I finish, the gate in front of you will open, and the dog will be set loose. There are explosives on a platform to your right. However, when I finish talking the platform will begin to move. What you must do is shoot it when it is closest to the gate. It will blow up the cage, killing the dog and revealing the next exit. Good luck! Oh, by the way, the sniper rifle's only got one bullet."
He finishes, and, as he promised, the platform starts to move. You grab your rifle and aim.