At 40 yards 00 buck has as much energy as a .22lr. Even if you hit it with half of a 12 pellet load, think of it as shooting a deer in the chest with a .22, six times really fast. Do people kill deer with buckshot, of course they do but it also wounds more deer than any other type of round. I would never use buckshot further than 25-30 yards, and only if the hunting regulations required it's use. If you're required to use a shotgun, do yourself a favor and use slugs. I've shot and lost quite a few deer back in New Jersey when it was buckshot only, but once they started allowing the use of slugs I recovered every deer that I hit.
As far as which choke to use. Buckshot doesn't work like birdshot in a choke, a lot of guys get better groups with just an improved cylinder. You need to pattern your gun with different loads to see which one works better.