The interface is simple, the controls are easy to understand, and the game is really fun. It has a good amount of tension, and it definitely made me want to keep playing.
One thing that could improve the experience is showing a short description the first time you pick up a new gun, so the player can quickly understand what it does.
I also found the UI on the left side a little hard to notice. Making it more visible and intuitive would help a lot.
Lastly, I think it would be even better if collected guns were added to your inventory and you could switch between them, rather than simply replacing your current weapon whenever you pick up a new one.