If there was no water available I would be more worried. You can go without food for awhile depending on your metabolism and body fat, but without water you probably would not outlast the zombie horde. It's easier to wait out the apocalypse with no food than it is with no water.
I would hope that some person or person unawares of the zombie horde outside the door to pass by close enough to draw their attention away and then make my getaway. I run fast, I am good at navigating over and around stuff quickly (not Parkour level good but pretty good) and I have a habit of taking stairs at 3-6 at a time. Otherwise I would try and clear a path in front of the door and try to getaway if the horde couldn't be lured or distracted away. As a last resort I would put one in my head as I was going down. I would rather not be a zombie, I have already been accused of being a Angry Young Man blindly following a crowd in another thread so I would naturally not want to be lumped in with another crowd, that of the Hungry Young Zombies blndly following the crowd to a food source.
There are some details that are too vague to do anything other than make rough guesses at what to do.
Are the zombie Rage type zombies (think 28 Days Later) or the traditional sluggish and uncoordinated zombies?
Is the room devoid of anything at all aside from me, the gun and the ammo? Are there any windows to the room.
What are the immediated surroundings? Urban? Suburban? Rural? Endless open terrain? Are there mountains neraby?
C'mon...provide some more specifics, mix it up a bit hehe.