The visuals, the sounds and music were all great. The health - ammo mechanic is really nice and fits the theme perfectly. Also liked the text shake before the last boss.
The beginning of the game felt too easy. There is a speedrun timer (which is a really nice touch) for better players, but still, I think the first half of the difficulty curve is too flat.
The bosses were great, the last boss was really creative and gave a challenge.
One small thing maybe is that most of the rooms felt super empty (empty rooms or empty rooms with a few walls). Some cosmetic props would have spiced the rooms up, like barrels perhaps.
All in all, I really enjoyed the game, my criticisms are minor, awesome job! :)
PS: This reminds me of Enter the Gungeon.