Thanks to all for the input and the ideas.
I have never measured the distances between fulcrum and lever edges in autos. Going by a rough visual comparison from memory I would say that most hammer guns have a larger radius between the axle (fulcrum) and thumb pad, than autos have between the hammer axle and the point of contact with the rear of the bolt.
I will also start comparing the springs of some real thumb buster hammer guns with the more gentle types. My observation is that quality is inversely proportional to cocking effort.
Hammerless actions probably were made with little regard to spring rate. The barrels doing the cocking are such an overpowering lever that not much thought was lavished on spring stiffness. Also spring making was probably a carryover from the flintlock days when the hammer on its way down had to carry the flint, overcome the frizzen and have enough power to rub against it to create sparks.
Looks like i got myself a project.
