ribit,
Any single-action designed like the Colt Single Action Army can fire if dropped on the hammer ... if it is resting on a live round in the chamber. That's why they always recommended that you carried it with only five rounds in the cylinder with the hammer resting on an open chamber. In '73, Ruger introduced a new model that utilizes a transfer bar to prevent any round from being discharged without pulling the trigger.
B.

The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence