jmr,
Your point on #4 is a good one, when I was in the service (Desert Storm) that was the load we trained with when we used shotguns, which was not that often.
I'll try a few shots in a representative sample (beef brisket from Wal Mart) and let you know how it works.
When I have used birdshot on close range targets, it has seemed to function as one single projectile, which was what I was looking for. The squirrel analogy is completely apples and oranges, because you are using a 15-20 yard shot as an example. That is at least twice the greatest possible distance I would be shooting. I too have shot small game at that distance, and have seen the same results.
At five yards, I penetrated a lawnmower hood (sheet metal) and blew up a gallon of water behind it with #1 shot. I also used #1 at about that distance and shredded a 2x4. (I really miss living on the farm in Arkansas.
)Come to think of it, that may be part of the problem, as I have my 870 loaded with #2, and some when they hear birdshot might be thinking #8 or 9.
I'll let you know about the beef brisket. Now, to see if I can borrow my buddy's lawn mower.
