You never said it had to be a C&R. So I'm going to have to suggest something far more modern, utterly impractical, and very very cool.
The Mateba Autorevolver:
http://en.wikipedia.org/wiki/Mateba_Autorevolver
http://www.youtube.com/watch?v=V0zxgyim_g4
If you look closely, you'll see it fires out of the bottom chamber.
I know you're trying to go a completely different way here but...come on...tell me these things aren't freaking cool.