Put tape over the receiver with an access hole over the screw. If you slip....... It may lessen damage. Do you have a clean slot on the screw? Put the gun in a padded vise if available. Tap as suggested.
As a toolmaker I sometimes take a punch that has a point on it and set it up against the slot ( or what ever is left ) far to the outer diameter and at a 45 degree angle. Tap the punch in the direction ( counter clock wise ) to loosen the screw. I have had pretty good luck with this in the past.