Chuck is very much on the right track. I'd add that pump and auto guns have firing pin travel in a straight line whereas most double guns utilize rotary motion within the firing train. If one knows the required impulse to ignite the primer (something the manufacturer would know), one can back calculate the required spring constant and travel to generate the needed impulse at the firing pin tip. Straight line force and travel are easier to deal with than is rotary motion. Older double gun spring stiffness and travel was most likely established by trial and error. Modern guns are most likely calculated, tested, and adjusted as necessary.
Lock timed depends on the whole train from spring to firing pin; weights, frictions, and travels.
DDA
