Ellis,
The rebound spring needs to be strong enough to enable the rebound block to return to its forward position quicker than the hammer falls to its fired position if the trigger is not held back. This is a safety item. To test: Aim unloaded gun at ceiling, drop a rod down bore (brass or wood), cock hammer. Jar trigger with plastic hammer or something similar by tapping on trigger guard until hammer is released. If rod is shot out of bore the rebound spring is not strong enough. If the rebound block beats the hammer, as it should, the hammer will seat on the rebound block and prevent the firing pin from contact with the rod.