What they said and the screw you are referring to is called a strain screw. It is adjusted to give sufficient tension to the main spring to reliable touch off the primers. A non-Magnum should have a minimum of 3.5 lbs of hammer tension and a Magnum needs at least 3.75#.
Loosening this spring will give the trigger pull a much nicer feel, BUT it may not work with your self defense loads. Once loosened, it can only get looser with every trigger pull. It should be bottomed out fully and tightened firmly.