The design advantage of a hand operated system like the hammer gun is the independence between the bolting and the firing systems.
There have been several bolting systems used in hammer guns, ie the Horsley round bolt, the Purdey underbolt, third fasteners, etc, while the locks remain pretty similar.
When attempts were made to turn the hammer gun to self cocking then you see a mechanical connection betwen the bolting and the locks. The Woodward unerlever is an example of this linkage. I once saw a Kerr that had a detachable screw to turn the underlever into a cocking lever at will.
