Really cool concept, impressive pixel art! I was unable to get past the first boss due to two things - not being able to shoot unless my ammo was full, and not being to suck in projectiles unless my ammo was empty. The second thing is a larger issue I think, because sucking up projectiles is your only means of defense other than evading the cans, which is pretty difficult. The game would probably be a lot more enjoyable and completable if the player could suck up cans at any ammo amount under 5. It could also be cool to be able to deflect incoming cans with your own can shots, as another means of defense. A good concept and good potential here, well done!