A Great Cannon or Mortar (not a Helstorm Rocket Battery or a Helblaster Volley Gun - weapons still too experimental for anyone to be an expert with) that an Engineer has joined may re-roll either one scatter dice or one artillery dice per turn. This, however, may not be the roll to determine the distance bounced by a cannonball - Engineers are good, but not that good! If the Engineer uses his re- roll ability during the firing of the war machine he has joined, he cannot shoot with his own missile weapon in the same shooting phase. Remember to fire all guess range weapons before any normal shooting!