GT,
Nice
I found that tinkering with the trigger spring itself can really have an impact on the quality of the trigger pull. My Mauser Parabellum has a lousy trigger pull and a relatively weak trigger spring. Swapped it with that of a Swiss 06/29 and the trigger pull changed dramatically. Never thought it would have such an effect.
Alternatively, another weird little spring with unexpected qualities is that of the extractor.
During our conversations with the former manager of Mauser Jagdwaffen GmbH, Prof. Dr. Rolf Gminder, he also reminded us of a patent he registered himself. Rolf designed a very elegant adjustable main spring. Basically the spring guide is equipped with andjustable (screw-in / screw-out) 'head' that can be adjusted with a screwdriver through a hole in the bottom of the frame.