Head, neck and broken wings are what I've most often been successful bring them down. With full feathers and down underneath and straight on or directly above shot through the breast is very tough to penetrate to the vitals. More often than not, I have best success with a crossing or even if they do come directly over head letting them pass a little past. In the latter case, the BBs go through the feathers better and into internals and still have opportunity to break wings.