AS2: How do I pass constructor arguments into an instance that extends the MC class?

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For example, I have the class Bullet, with a constructor:

Bullet(start:Point, target:Point){…} // constructor

But, if this Bullet class extends the MovieClip class, how do I pass start and target into it? Right now I am using attachMovie to create instances of the class. (They behave correctly with the static properties set by the constructor, but to be useful it needs arguments.)