Interesting. I've been giving myself fits this year because I keep crippling birds that are going straight away. Every time I shoot at those birds its seems like one leg drops, second shot other leg drops, but they don't come down. I thought I was shooting low but maybe I'm just not getting enough penetration. These aren't far shots either.
Earlier this year I hit a bird going straight away at 20 yards first shot and dropped one leg, second shot was at 35 yards dropped the other leg, then third shot was at 50 yards and blew more feathers off it and it flew off into the distance.
I'm shooting 3 inch 2 shot steel so there's definitely knock down power there. Anyway, quartering away shots are the hardest for me. I find crossing shots to be easy and I grew up pass shooting ducks and geese so when a bird comes at me it feels natural.