I am active-duty Navy, and we just got a message regarding an accidental discharge with a Mossberg 500.
Here's a reference from the Naval Safety Center:
To sum it up, shotguns of this model put together before 1994 had a different firing pin spring. They should be replaced with a new part to avoid a shock to the weapon from causing a discharge, even with the safety engaged.
Here's a link to the Mossberg Customer Service / Parts page (USA):
I would contact them to make sure you are getting the correct part. Don't do the repair yourself unless you are sufficiently expert and willing to accept responsibility, etc.