What choke came with it? If it's a mod, I'd pick up a box of 3" #4 or #5 Rem hevishot and hold shots to 35 to 40 yards. If it came with a turkey choke I'd get 3" #6 copper plated lead and hold shots to 40 yards.
I primarily use an NEF 28" mod 12 gauge fixed choke that consumes 3" #4 hevishot that tested good to 40 yards (haven't tested farther). My Rem 870 supermag with modified choke also patterns this load well out to 40 yards (again, haven't tested farther so I don't know the ultimate range). For long range work my 870 supermag wears a .665 extended or .655 extended ported choke and I use 3.5" Win high velocity #4 or #6's and it'll hold a great pattern out to over 55 yards.
But since you don't have the time to pattern it with different loads/chokes I'd use a more open choke, mod or full, and hevishot and call them in closer and be conservative on the shot selection.
later,
scruffy