1) Make a bar. Select it, hit F8, and turn it into a movieclip. Make sure you hit out of those nine little boxes, the left middle one.
2) On the properties panel, where it says instance name, change it to myBar.
3) On the frame where the bar is located, click the frame. Open the actions panel. Script:
onEnterFrame = function {
myBar._xscale = hitPointsVariable;
}
Now anytime you change the hitPointsVariable to a number between 1 and 100, it will change the scale of the bar to that corresponding number.