I had a similar thing happen 2x, both on pheasants.
The first time, I had a bird get up about 10 yd away and I was very quick on the shot. It folded with some odd feathers floating down. On picking it up, I noted the whole 1 1/4 oz of #4 from the modified barrel of my 12ga had hit it squarely in the vent and it was totally cleaned out. No pellets anywhere else in the bird.
The second time, another pheasant got up close and I hit it between 10 and 15 yd away with 1 oz of #6 from the modified barrel of my 16ga. It had most of the neck gone with only the trachea holding the head on. One pellet stuck in the skin of the back where someone using #5 plated had wounded it.
Happens.
