Good job on the game, it's actually very nice to play. You could work on the UI, to show the player how many bullets he has on the weapon closer to his character, just like in Enter the Gungeon for example, I hadn't noticed you had to reload your weapon until I kept pressing the trigger and nothing happened, since the ui blended itself with the background. Also on this subject, you could make the weapon auto reload itself if the player shoots and the gun is empty, most shooters have it. You could have done it for realism, but i think it would make the game more fast paced. You also could make enemies not spawn close to the player, sometimes the zombies would spawn right on my face, for some sweet free damage, and since I had no idea where they would spawn, they would cash on it.
I hope to see you expand the game, maybe add bosses, I think this game has a lot of potential.