Overall Great game!
Art was fantastic and the subtle changes for breathing and movement were nice. Music was a good choice as well as the sound effects, though the balance is a bit off... the gunshots are too much stronger than music and are a bit off putting on the ear. Sam with the zombie noises considering the distance they felt too close all the time... but if that was just for ambiance It makes sense.
The use of the theme was all right but I would have preferred something a bit more then just shooting two sides... perhaps putting the two gunners on a rotating platform and allowing them to swap positions would have been a better use of the theme?
The bullet drop randomization was a bit high on the machine gun considering the distance and that made aiming harder. Also clicking on the ammo dropping into the middle of the map seemed odd to me considering their position was stationary and the plane should have just dropped it on them. I though it was like terminator and I had to shoot the ammo box and as a result I ran out of ammo. It wasn't until I played again that I saw the "click on ammo" but I feel like that would also fire a random round in the air, which is counter intuitive a bit.
Still, all in all I enjoyed it and it looked amazing!