I've never seen 7.5 give good results past 25 yards or so.
Size 6 will get it done to 35 with no problem, especially in a 20 where it keeps the density up.
Size 5 gets it done period.
Size 4 will kill birds at 75 yards from behind, as I've seen it done numerous times. But there is not as much density, and you probably need to be choked up full. There seem to be just as many runners with size 4 as with anything else.
My uncle used to swear by size 2. He claimed that one pellet is all it took since it always penetrated the bird completely, and that seemed to incapacitate them instantly. I haven't tried it myself.
