You could always use the fastloop extension to ensure only one bullet is ever required...it can instantly make the bullet hit a target, or nothing. That's no good if you want to be able to see the bullet though.
look just make sure all bullets get destroyed when closer than 0 pixels to windows edge and make sure the bullets and all other objects are destroyed when there animations are over.
you can also make a counter that always shows the number of active objects in the level.