RE: Buckshot
Buckshot.... I hunting in a club where we dogged deer for years, rule was buckshot only. Most of the deer I've killed have been with buckshot, followed by muzzleloader. All that said... it is a real short ranged proposition. If I knew I had only one shot... and the deer was broadside... I'd say 40ish... further out it gets dicier. I've shot and killed further, but I emptied the shotgun, too. I found that WInchester Supreme (black box) in 00 and 000 was tightest patterning in my 870.
My humble advice... pass on buckshot, if you can, or unless you're constained by rules, or are still hunting (that is, walking & jump-shooting) in really, REALLY thick stuff. At 30 and under it's lethal - get past 40 and effectiveness drops off quick, unless you're a good shot AND will get some followups. Buckshot doesn't penetrate that well - you won't get pass-throughs unless REAL close - and there may not be much blood.
Enjoy the hunt.