ditch that approach and use the drawing api.
Originally Posted by lancerawks
as3 is nearly identical, but preface your drawing api commands with 'graphics'
// myMovieClip.lineStyle ( thickness, rgb, alpha )
bullet.lineStyle ( 1, 0x000000, 100 );
//myMovieClip.moveTo( x, y );
bullet.moveTo ( startx, starty );
// myMovieClip.lineTo ( x, y );
bullet.lineTo ( targetx, targety );
// then, whenever you want to clear it:
myMovieClip.graphics.lineStyle (1, 0x000000, 1);
I hope that is enough info to help you. The drawing api is pretty straightforward to use, just experiment with it until you achieve your desired results.