Make them “disintegrate” into the past where they train and become stronger until they meet back up with the main characters for revenge.

If you make them go far enough back, they could start a whole historical faction that has a cult about a coming apocalypse started by those who can send people back in time. The cult’s main objective would be to destroy the gun by allying with some time shift immune entity.

The players won’t know what is going on until they find a temple with “A symbol of the apocalypse” which is just an image of the disintegration gun. Bonus points if they discover this after the gun has been destroyed and the cult celebrates for a seemingly minor victory.