At 20 ft a load of #5 shot or smaller is still extremely lethal. At 20ft a head shot with #5's isn't going to leave much more than an empty brain pan.
I'd ask why you were using #5s when #7 1/2s are plenty to penatrate the head of ducks and geese. but I know the answer.
Sure if I get to pick the scenaro where the BG is going to stand there and let me shoot him in the head or maybe if he promises to stop if he gets a nasty flesh wound birdshot would work every time. Hopefully your real life will match the scenaro you've planned for.
Since some of the knoledgeable folk on this forum have schooled me on flightcontrol wads OO buck will take care of my shotgun SD/HD needs.