Using 8mm lead balls, they are quite effective but no adult should use one for hunting.
I remember my mother buying a new kitchen when I was a boy, that she saved up long and hard for - we were as poor as mites. As soon as she put that anthracite burning stove in, I thought of how it can be used to smelt metal. A week later she went out for an hour, and I had a two part plaster of paris mould waiting with two pounds of aluminium to make an even stronger catapult. Alas, in the rush to get it done before she got back, I did not bake the plaster of paris to drive out the remaining moisture, so no sooner than I had poured this monster load of aluminium into it, it exploded, putting molten ali all over the new vinyl floor (in vogue then), and embedding it into the new kitchen units.
That memory is more painful than the welts on my fingers and wrist from when balls shot in a hurry hit the yoke on the catapult.
Putting that aside, catapults are poor for hunting and I would not use one as an adult for that as it is unethical. Even lead balls cause more injuries more than they kill, as whatever the Youtube can mislead you to thinking is possible, catapults are not consistently accurate and do not have enough power: an air rifle does better, or far better still a .17HMR. If killing animals to remove pests or to eat, do it humanely.