Others have already hit the nail on the head. Definitely pattern the gun using as many different shot-shell buckshot loads as you can afford to buy or scrounge up. I'd try say #1, "O" and "OO". I'd check the pattern at say 20, 40 and 60 yards. This will give you an idea of the "coverage" you would expect on deer sized game. I'd alos add my advise to limit your shots to about 50 yards, regardless of the density of the pattrern.

My advice would be the same as above, but it you want to shoot a even tighter pattern (hopefully) you might check into getting a extra full or 'turkey-type'choke that will hold a tighter pattern at longer distances. I routinely see guys harvest deer out to 80 yds with shotguns setup to only shoot buckshot.