This was really good! The concept is really simple but it has a lot of mechanical depth. I especially liked the times the game forced you to change lanes by placing keys after the locks.

A few nitpicks though, because I feel the game has a lot of potential:

-It was kinda hard to tell apart 2 and 1 notes when they appeared in quick succession, I feel like they should be easier to tell apart at a glance (giving them some difference in color or shape?). 

-I feel like the guns fire just a little too quickly, so maybe it'd be nice to make them more predictable through either making them slower or making them fire in a patern (fire in lane 1, don't fire in lane 2, fire in lane 3...)

-The 3 and 4 notes can be hit multiple times in the same lane if you mash the button and I feel like they'd be more interesting if they forced the player to change lanes instead of just straight out murdering the down button.

-It can be kinda hard to see in which lane the ball is when you're paying close attention to the notes so maybe a color change or some other visual effect could make it easier to tell.

Overall the game was really impressive both with its premise and the execution. If you polish it up a little and make some more levels (maybe let notes come from both sides at the same time?) it would definitely be something I'd be interested in!