With all ur birds u've shot, i persume u skin/pluck/butcher a few for urself.
Have u ever noticed using 5's or even 4's a pellet to penetrate right throu feathers, the breast muscle and breast bone to hit the heart/lungs? Ie prove to be a fatal wound.
Over the last few years i do look for pellet damge and wot pellets hit where and how far they penetrated, very rare to see pellets travel that far into breast muscle far enough to be fatal.
My opinion is after 20m the only way u will kill a bird is a head shot, 1 or 2 pellets is all it takes and they will come down stone dead, when doing walked up i have seen birds hit hard/borderline pillow cased even when a bit further awy but caught full pattern not to be found by the dogs, my spaniel got 1 that ran 50m a few seasons ago, no one could beilieve it was not dead the way it was hit, just luck we found it really
When u strech the ranges a 6 wil still be enough to kill a bird in the head but will not be enough to break wings or legs, which wil not be pleasant for bird but i would guess it will recover, with ur larger shot sizes they will still break bones at longer ranges. But do i really want to be breaking bones at longer ranges, wot % of birds are picked up that have a wing/leg down some can cover massive distances esp when started of fairly high