I used to shoot skeet with a guy who used popcorn as a filler. He had a Model 12- 20 gauge with a Cutts. He used to take a one ounce load, cut it down to 1/2 ounce and fill the rest of the hull up with popcorn. About the best skeet shooter I ever saw. While I was struggling to break 22 or 23, if he missed more than a shot or two in four or five rounds, he was having a bad day. 😄
Did he pop it first?
I know guys shoot all kinds of things as filler. One guy like amaranth seed. Of course, some types of amaranth are huge agricultural pests, so I don't recommend it, but I try to stay away from fillers.