DSII - Troops Dug in?

Well here is something that is very confusing. The rules for dug in   
infantry are a little confusing. They are clear when it comes to   
artillery fire and infantry fire, but what about vehicles? I would hotly
  
debate debat that an autocannon is less effective than a DFFG against a 
 
bunker/Duging position? A larger ball of super hot material striking the
  
ground would cause a rather large explosion! Maybe I am confused?