I think the game's foundation is good, but would have more potential if it was actually properly designed. I don't know if it's supposed to be a rage game but the gameplay did get frustrating just because of how strict every challenge is. The biggest problem I have is with the bullet count mechanic. I think it's a fun mechanic itself and requires you to be careful while aiming but it gets to a point where requiring 100% accuracy for every room doesn't feel exciting or gives me the pressure of counting down to me, it just makes me feel annoyed. Giving a buffer of ~10% or even ~25% would make the game a lot more enjoyable since it'd still have the challenge of not requiring precise inputs.
If you do want precision to be the selling point of your game, you should at least make dying and respawning WAYY faster. Think of Celeste where the whole point is to be precise, but because dying and respawning take so little time it doesn't get (as) frustrating. You can try doing that if you really want perfection and precision to be your game! I appreciate the checkpoints though, it made the game better. Or even some of the mario Kaizo games, those not only have a fast respawn timer but also well thought-out mechanical inputs that make it fun.
The artstyle and audio is quite nice. I like the voice acting a lot and it gives a lot of personality to your game! I still think your game is quite polished and well-made and the foundations of a great game (whether or not it's a rage game) are there, I think more thought into game design will make the game shine even more.
I hope you don't take these comments too harshly. I think the core gimmick of the gun levels is unique and has a lot of potential, just needs some more thoughts on the designs in my opinion!