While your 40 yard range is probably unrealistic with a cylinder bore, I have killed rabbits with a 12 gauge #8 dove load. I was upland hunting at my local shotgun-only state wildlife area, and the bunnies were out and about, and in season, so I took a shot at about 25 yards (IC choke.) Peter Cottontail flopped over, kicked around in a circle twice, and died. Bunny #2 was crossing the trail about 20 yards in front. He never made it to the other side.
If I was going small-game huntig with my shotgun, I'd probably use a #6 pheasant load, but the #8s were what I had, and they did the job. Oddly, I've actually gone back to 7 1/2 for my upland loads, they seem to kill more reliably than the #8s, at least for me. Last season, I used the 8s for the first time, and I had to twist more birds than I have in the past with the slightly larger shot. I'm thinking that shooting at a stationary rabbit allowed more pellets on target than a 45 mph dove.
--Shannon