A 1 1/2" bore punt gun would use up to 3 pounds of #3 or #1 shot for the duck meat market. I do not know what the powder charge was, however standard American #3 shot count is 108 to the ounce, 72 for #1.
The record for ducks killed with a single shot, is I think, 128. The range for the flock was 90 to 110yards from the punt.
The gun was aimed some 4' above the water at the middle of the 'flock', so the birds, startled by the smoke cloud, would rise into the shot cloud passing over and of course, into the water as well, hitting those still sitting on the water.

Do the math.

108x16 = 1,728lb x 3 = 5,184 #3's to 3 pounds.
72x16 = 1,152lb x 3 = 3,456 #1's to 3 pounds.

Quite a cloud of shot.

Very impressive.