Firing Pin Spring - fits all models (end with the most coils goes toward the spring cups).
Part number 10 on the Glock Schematic.
This "reduced power" spring will provide a smoother/easier trigger pull. It should be noted that the 4 lb spring can cause misfires so if it is used you MUST prove its reliability and shoot several hundred rounds without incidence. The combination of this spring with a 6# trigger spring can possibly cause trigger reset/safety engaging issues. If this happens you'll need to swap back to either a 5# trigger spring or firing pin spring.