Very cool concept and good execution, and I'd definitely recommend continuing this into a bigger game.
Fixes:
-Work on the collisions a little, because getting your corner stuck on a wall can be a little annoying.
-When you die, the bullets stay where they were when you died, which can be especially bad if you died in a really rough spot. Maybe respawn with any bullets you'd already collected (minus any you'd put in the holders).
-I think I broke the game a little while I was in the last level by getting killed by two of the green enemies at the same time. I respawned as two players.
-If you die while pulling a bullet toward you, it will continue going in that direction, which can make it end up in a far away dangerous corner.