I've seen a lot of deer killed, even killed a few myself, and all but a couple of them were killed with buckshot. Within it's range limitations, it's deadly. I always favored 3" No. 1, out of a modified choke barrel. Barrel length doesn't matter too much I don't guess, but all mine were 28". When I started hunting, I didn't know anything about how to pattern a gun, so I just set up a bushel basket and stepped off 40 steps, and shot. (Sears/Mossburg 500) I counted 17 out of 24 shot in the bottom of that basket. I figured that was good enough.
I had a Remington 1100 magnum with a 30" full that shot 000 pretty well, but I never killed a deer with it. I did miss a deer with it once at about 10 yards. Put the entire load of 000's into the side of a little tree that jumped in the way. You could have covered that pattern with a baseball cap.
I have seen a lot of deer killed with 000 though. It breaks bones like they were matchsticks. I don't think I've ever seen a deer killed with 00 for some reason. I guess it just wasn't very popular with the guys in my hunt club. For the most part it was 000 and No. 1. You think the 9mm vs 45 debates are something, they got nothing on the 000 vs No 1 arguments.
I would guess the longest shot I ever took on a deer with buckshot to be about 35 yards or so. The closest was about 10 yards. (Deer driven with dogs.) I personally never had a "one shot kill." One shot knockdowns, yes, but they all required a "kill shot." Usually a load of birdshot to the back of the head.