To throw out anothers opinion.....
I shot a 12 ga for about 5 years for basically all of my hunting. I decided 3 years back that I would give a 20 ga O/U a shot. Since then I don't think I have shot an upland bird or dove with anything else besides a 20 ga. I do use the 12 ga in the duck/goose blind, but with upland the 20 ga definantly gets the job done and then some. It's patterened well and for the most part if the dog points the bird or we run them into a corner, (the most likely scenario when a bird is shot) the bird shot is usually dead on retrieve or busted up enough that he has no intention on running..usually dead though. But yes there are sometimes birds are hit and need to be ran down by the dog .410s-10ga will all cripple sometimes... I've found that the birds that get away are usually the ones that are missed close, and end up with a broken wing at 45yds....it happens alot with young shooters, which is the majority of people I hunt with. I've found in my 9 years of Firearm hunting (not counting the BB gun days :roll: )that if I take my time on those birds that bust out from point blank under the dog and myself, and then make a careful shot..the bird is dead on retrieve. Just for reference..I am referring to WILD BIRDS. I do help out with the dog at the Pheasant preserve, maybe shoot a scratch bird here and there, but there is a huge difference when it comes to clean kills and retrieves when comparing pen raised vs. wild.
FYI....The majority of pheasant shot and found are taken within 25-30 yds of the hunter. Minnesota Sportsman (if I recall correctly but maybe F&S did it) did a study on birds lost and shot during a variety of different scenarios. It is a great article if someone finds it somewhere post some of the data. But if the 25 yd majority statistic holds true.........then a 20 ga or likely 28ga will definantly handle your needs....
What ever you can hit with should suffice.
And consider the fact that its not really that much enjoyment to shoot birds way out at 40+ yards..I just can't say I enjoy it as much when a bird flushes wild someone takes a long range crack at it and then it comes back with a 1 bb hit to the wing and basically alive and well. Just don't feel like thats hunting. I must say I would rather hunt with the dog and let him find the birds and then make a shot. When I think about hunting I want it to be just an enjoyable time with family, dogs, friends....Crippling and losing birds doesn't help the mix. We (or just myself) go out and enjoy ourselves and try to get the ones that get up close. If they get up early and flush then I guess he gets away..not a big deal. Just keep walking and enjoying what we love.. Isn't that the purpose for most of us, Enjoyment??