as you can see in the attached file, there is a field called 'HARDNESS' (did you feel it slap you in the face?)
although i'm not sure how it works, my best guess would be that 1 is the default setting, 0-0.9 would be weaker, anything 1< would be harder than default.
although i think its the objects hardness, not the type its set as, so fighters should go boom quite nicely