its really not all that hard to imagine a handgun where the hammer drops to the bottom cilinder rather than the top cilinder. i dont know why youd make a gun like that (probably something to do with the plot) but to say its nonsensicle is a bit of a stretch.
The Chiappa Rhino is another model that fires from the bottom cylinder. The reason it's not more common is that the mechanism tends to be considerably more complex. A revolver isn't as simple as it seems from the outside.
