A resizing mallet would work in a soft metal like gold, Im not sure if you can resize that way an aluminium ring. Yesterday I made a titanium ring for my wife, Im 100% sure a mallet would be of no use in it, she asked to make it a tiny bit larger ID, I will put it in the lathe again.
The ring OD is exactly 19mm (almost 3/4") so having an ER40 collet for that size was a real saver.