If a Master Engineer is not fleeing, a single war machine within 3" of him can use his Ballistic Skill and re-roll one artillery dice during the Shooting phase. This cannot be the dice that determines the distance bounced by a cannonball. You must nominate which war machine, if any, will be using this rule each Shooting phase before any war machines within 3" of the Master Engineer are fired. A Master Engineer cannot use this special rule and shoot with his own missile weapon in the same Shooting phase.