@emo_girl,
When a missile is launched from a moving platform, the speed of the platform is added to the missile's, which would increase the missile's range and the increase is significant. What we have are 'forward-quarter' and 'rear-quarter'. The 'forward-quarter' is the flight direction. If the target is unaware of the threat, then in theory 'forward-quarter' range could increase by as much as %100. The problem is that there is no way for the aggressor to know for certain if his target will remain unaware of the threat. Once the victim is aware of the threat, he will maneuver and as the missile maneuver to match, the missile will lose speed and fuel. As if that is not bad enough, if the victim does have reserve speed, meaning he is not flying at his max, once he is aware of the missile threat, he would increase his speed, thereby negating any increased range the missile may have from the launch aircraft.
This is why no sane pilot will fire at the missile's maximum capable range, even if he is already supersonic. If the aggressor is at a higher speed than the victim, the aggressor would, or should, use his currently superior speed to bring both aircrafts inside the missile's maximum capable range. The deeper inside this range, the greater the speed and fuel reserve the missile will have to bring to bear against the victim. For example...If the missile is capable of 100km maximum range, the aggressor should position himself and the victim to be within 75km of each other before launching his missile, even if the theory tell him that he could have as much as 200km due to initial launch speed. It gets worse. If the victim can out accelerate the aggressor, meaning there is now a target-over-shooter speed advantage, and if the victim climb to gain altitude, he will force the missile into further disadvantage as the missile now must fight against gravity. Once again, it does the aggressor no good to launch at the missile's maximum capable range.
This is why aggressors usually try to be at a higher speed than the victims, creating a shooter-over-target speed advantage, be at a higher altitude to deny the victims the use of gravity, close the distance between the two aircrafts to within about %75 or less of the missile's maximum capable range, then launch. This is regardless of how much more range the initial launch speed can give the missile. The fact that the target can maneuver in three dimensions seriously complicate the shooter's launch decision.