Fun game. I took me a bit to realise this is kinda a deck building-style thing, you're not getting an upgrade so much as adding a special shot card to your deck that'll be randomly distributed each draw/reload (hence why "remove bullet" is an option, its for removing weaker cards from the deck so they won't be drawn). Good idea I have to say, unfortunately the waves of enemies are more identical, so the difficulty doesn't scale as you upgrade more and more.