I'm pretty sure the stories of punt guns killing dozens of birds in one shot are just that: stories.
You could spend hours quietly paddling into position, maybe trying to herd ducks together for just one shot before they've all lifted and gone and you have to reload. Those one shot bags I've heard of are typically in single figures. Thinking about the range of shot, and the type of pattern such a gun might throw, superimposed over how a raft of duck might look at 50 yards at low elevation, that makes sense to me. A shooter with an auto, a few hours on a good flight path and a steady supply of cartridges to hand will do more damage.
