In between being Humble and making games about dangerous rabbits, the handsome folk at Wolfire have made a game with astonishingly complex gun-mechanics. It's called Receiver, and it's arguably one of most interesting experiments in the first-person shooting field for some years. Rather than clumsily attempt to precis this, I've posted Wolfire's own mechanics video below. The game has just had a big content update as it finds itself upon the Steams, and there's a video explaining that, too.
What a lovely day I am having.
Wolfire put up a post-mortem of their experience with Greenlight just here.