I haven't played something quite like this before, but it felt good. I beat about 20 bosses, but after upgrading my max ammo and ammo refill rate, I felt like the game didn't raise in difficulty to match the advantage I had. So the bosses began to feel easier and same-y. I think I noticed they were attacking more often though? Good concept. Good execution. Needs just a touch more variety in difficulty. Good job.