What is hammer spring adjustment?

The hammer spring controls how hard the hammer hits the firing valve, which controls how long the valve stays open and how much air is dumped per shot.

On many guns this is user-adjustable. Heavier hammer strike opens the valve longer for more power, at the cost of air efficiency. Lighter strike yields more shots per fill at lower velocity. Hammer spring adjustment is a common first tuning step paired with regulator setpoint changes.