A lot of potential here. The game is interesting overall, and aside from a few ammo bugs, proved very entertaining.

Pretty good, man. If you could make the ammo blink before it disappears it would be awesome (just to simply indicate that it will disappear momentarily). Good job.

It's a nice clean game.
I wouldn't of added a crosshair as it makes it a tad too easy and as the ammo drops are timed you should be able to pick them up at any time, this will mean that players have to time their bullet use and pickups so they don't run out of ammo.

VERY linear.
The zombies are just as fast as you and don't change direction that quickly, so it's quite easy to avoid them. That, added to the fact that you can one-hit everything (and with the size of the zombies, you almost never miss), makes you practically invincible.
There are so many things that could be easily added in later levels to make this much more enjoyable game. Different kinds of zombies (e.g. faster ones, exploding when shot, more durable ones,...) more weapons (weaker longrange (rifle), stronger shortrange (flamethrower, shotgun), something that pierces through everything in its way,...), obstacles, etc.

But with a working, fluent mechanics like this, it would be almost a shame not to make it into a larger game - e.g. something with a story - for example an adventure in a zombie infested city, survival horror, I don't know... I would gladly give five star rating to any more complex game based on this mechanic, because now it is just too linear and becomes boring quite quickly. The competitive, score aspect isn't that fun, as the final number depends on everyone's patience more than on their skill.

this game is quite good i would have like to see a good story line added and maybe a few guns like a shop. also more textures like grass dirt ect instead of stone the whole time but otherwise this game is pretty good i do like zombies and shooting genre games so this is good hope its a save and NOT a blam