The Delta Eagle Crossbow from Nanao's Vampire AU fanfic.
In her story, itís actually a specialized vampire killing firearm that uses the modified wheel design to hold additional rounds. I had a lot of difficulty incorporating the large wheel into a gun design. In guns like revolvers, the barrel rotates perpendicular to the body of the weapon. After a few failed designs, I gave up and drew a crossbow instead. That way, at least the bow string loop around the wheel to give it some extra oomph.