A solid simple mobile game :)
The core gameplay idea is sweet. My current highscore is 24. The only criticism I have is, that the next circles were often spawning in inconvinient spots, were you had to rotate almost 360 degrees, until you could shoot it. If that happened too often in a row, which often was the case, it made the gameplay overly slow at some points.
The music had a nice touch, and fed the game with a lot of ambience. Though, in my opinion it did sound a little repetitive and unfittingly dramatic and melancholic. The latter might be intentional though, It'd be a legitimate design decision.