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The missile ability causes units to explode on contact with the enemy. This explosion is done during combat. One combat round will be processed (so a chance the missile itself could be hit for anti-air units you create [see below in notes]) and then the missile will explode. The nuke range ability has been enabled for non-special nuke units when they have the missile tag. This means your missile can be nuclear as well to create tactical nukes.
NOTE: If you give a missile unit nuke abilities by change nuke range to: <iNukeRange>1</iNukeRange> the unit will become a nuclear missile. In this way by defining the air range and air bomb abilities for the unit you just click on air bomb mission and select the target in range. Your unit is now a tactical nuclear missile.
NOTE2: Since one combat round is processed before the missile explodes, you could create your missile unit, then the anti-missile unit with a combat bonus against the missile unit. By testing and getting the correct values the anti-missile unit has the chance to destroy the missile before impact. Example: You could have a ballistic missile and a SDI unit with a combat bonus against the missile. The SDI unit has a chance to destroy the missile before the ballistic missile explodes.