Curves in action script

Hi,
Okay, I have a box with curved sides. The idea is that the box starts off squared and expands to a certain size but I want the sides to curve out and snap back elastically. I can do a simple tween to get the curve, but it doesn’t have the elastic effect I’m looking for. Can I control the arc using action script? Any help would be much appreciated.

Thanks,
AJMoore