The game is quite challenging indeed! The gameplay is simple to understand, and the movement feels nice too. I'd say 18 bullets for the handgun and 6 for the shotgun is quite harsh, I think you should ease in the difficulty a bit, especially at the start of the game, just to let the player familiarise with all controls first. Also, I think the ammo refills need to be more prominent. Larger and flasher icons would be nice, just to promote the fact that this needs to be collected!
Anyway, I think you've done a nice job making this awesome game! Well done!