Interesting question: how would you handle a mass combat against bullet-proof Autons ?
At first glance, I'd rule that offense rolls using firearms only will automatically fail every time.
Any other angles on this?
I'm not a big fan of automatic failure - I'd simply go with not allowing the firearms' damage rating to modify the Offence value. So a team of UNIT soldiers with assault rifles would get their Co-ordination and Marksman values added in, but wouldn't get the Good damage bonus.
(The system is abstracted, so they could still be hurting the Autons through indirect fire - blowing up fuel tanks, forcing the Autons back into hazards, etc..)