Liking the idea of this and can see it being a good game fully fleshed out.
A few things I thought didn't work too well or suggestions I think would make the game better:
The way the enemy objects bounce around when you hit them looks odd to me. the way they randomly instantly move and change angle. I think it would look a lot better with a different effect such as bouncing the opposite direction the bullet hit it slightly or gradual movement insteat of instant.
As medipacks and health etc reach the bottom bit they seem a bit too fast for me. Maybe lowering the max fall speed of them a little.
Even when collecting every ammo with two guns equipped I ran out before there was more ammo to collect. This makes the gameplay a little frustrating when waiting for ammo. Especially if it drops in an area you have to charge into tons of enemies for.
Apart from those things I found it enjoyable to play.