Role of the revolver? What truth about the revolver is revealed later in the play?

The presence of the Serbian's revolver evokes fear in Raina, but it is unloaded, thus, not dangerous. I'm not sure what you mean about the "truth" about the revolver? Please add details.

The role of the revolver is that it allows the man (Bluntschli) to prevent Raina from calling for help ,I.e,keeping him safe.later on it is discovered that revolver had no bullets in it because he carried chocolates instead of cartridges.