Shotgun primers probably require less force to actuate than a valve stem. But, shotgun hammer springs are diabolically overpowered. It only takes a comparison between a traditional V spring and a modern coil spring powering the hammer of self loading shotgun to appreciate the range in spring power available.
The Giffard had an external hammer. The design was abandoned and I wonder why. Complexity could not have been the issue, the system only has four parts, hammer, spring, guide with rebound prongs (if a coil is used) and sear. If a V spring is used there is no need for a guide, the spring itself provides the rebound arm for the hammer.
A friend owns a Giffard rifle. I will have to pressure him into letting me take a close look.