It's a bit boring after a while, but it can be improved to be a really good game.
-Some more randomness in discs speeds and directions.
-Perhaps some kind of disc which cannot be shot? You know - only most of them have to be shot, but others will give you a penalty (losing lives, points, etc.).
-Sounds are a bit annoying... The best way to improve that, in my opinion, is adding unlockable, purely cosmetic weapons which change only sounds of shooting and level ending. Default (current) could be named flash gun, other unlockable could be shotgun (with its shooting sounds and reloading sound at the end of level) and silenced pistol (with simple non-distracting sounds).
Anyway - i hope you'll improve it, because it looks like you put some effort into it and you're able to make a good game from it. Good luck!