fast-eddie,
You're probably thinking of a percussion gun where the hammer physically strikes the cap. It's a little different with cartridge guns: After the firing pin strikes the primer, it rebounds back behind the breech face. Otherwise, it would rest in the newly formed primers indentations. The protruding pins would make opening the action difficult, possibly damage the firing pins, and make loading an exciting adventure. Typically, in modern guns, the hammers are stopped short. The hammer strike imparts enough inertia to the firing pin to ignite the primer. This may be why the Rossi's hammers don't seem to go fully down, there's a gap to give the firing pins room to rebound.
There's another scenario, some coach guns have been made with "phony" external hammers. They aren't truly hammers, but are cocking levers (made to look like hammers) for the internal "true" hammers. This is understandable because guns with true external hammers don't perform well when subjected to a modern "drop" safety test. Many old-time-looking modern hammer guns (Ruger SAs, Colt 1911s) have transfer blocks, or other devices to make them drop safe (or safer).