You are talking as if the hammer itself has to hit the bullet all by itself. That would never be the case in a bottom barrel revolver (at least not in a practical one). The hammer that you see would act as a device for user input and as a visual output that the gun is cycling. The mechanisms involved to make this happen would not be too hard, but would be complex in comparison to most revolvers' mechanisms. Definitely agree with some of your op though, if the revolver uses a bullet cartridge, it isn't going to make it into the barrel.