It sounds like the trigger rebound spring is WAY too light. The trigger can't get out of the way of the hammer falling because it hasn't been pulled far enough to the rear. A heavier spring requires more force to release the sear engagement and the trigger will automaticly move farther back allowing the hammer to miss it.
You should also check for "push off" of the hammer as this will cause the same problem. Then you probably will need a new trigger, hammer or both.
Good luck;
Mike

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Mike
S&WCA #2470