Elaborate.

Hammer hits top. A transfer bar transfers the energy of the hammer striking down to the bottom of the cylinder. This causes pin to hit round, gun goes boom, the bullet goes out.

Which might make sense... if anyone ever used that design ever... But there are very few examples of revolvers with lower barrels, and they all either use a low-set hammer to match, or an internal cam system to fire the weapon instead of the hammer. The problem is that doing it the way you describe is unnecessary. It just adds more parts for mechanical failure. Though it certainly wouldn't be impossible, it would be superfluous.

That's why there aren't many revolvers as such. I just pointed out its possible, not practical.