John Browning's First Rifle Design Double-Action?
Somebody may have to correct some details in my recollections, but I believe St. JMB's first design for a rifle for his brother involved a simple trigger as follows (forgive the hasty sketch in advance, please):
The drawing on the left shows the one-piece trigger/hammer combination in its cocked position. The arrow represents the force of a spring against the trigger/hammer. The dot with the "S" is a stop on the trigger's movement forward.
The force of the spring, being below the axis or hinge ("H"") of the trigger/hammer, tends to rotate the trigger /hammer clockwise against the stop.
As the trigger is pulled and rotates counter-clockwise as shown in the right-hand drawing, the force of the spring at one point is suddenly above the axis ("H") and suddenly rotates the whole trigger/hammer counterclockwise, firing the cartridge. Kind of gets "overbalanced" at that point.
I don't know. Would that be a "double-action" rifle?