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With center fire guns the striking action on the firing pin propels the pin forward where it would normally strike a primer to retard its momentum. Without a primer or snap cap to cushion the stoppage the pin travels fully forward and the tip momentum reaches higher velocity and a sudden stop. The compression action of the primer would normally slow the primer and put it under compression stress rather than elongation stress. One reasonthe S&W hammers were always less susceptible to breaking pins was the pin being loosly held to the hammer with a rivet which allowed some stress energy to be absorbed by the floating and rotational action of the pin. When the pin is part of the hammer as with the older Colts the elongation stress is not dissipated with the rotational action. Hope I made that clearer than mud.

Of course with rimfire weapons the firing pin would strike the rim of the cylinder potentially peening the cylinder or either breaking or peening the firing pin.