For you first plains game I would use a scope but it is up to you and you maybe left with a bad taste if the animal of a life time is spotted on dark or at 250-300 yards.

Shot my
Warthog at 40 yards
Impalla's at 200 yards,140 yards and 175 yards
Zebra at 120 yards
Kudu at 160 yards moving
Baboon at 160 yards
Hyena at 45 yards
and Cape Buff at 25 yards.

I think if you can hit a balloon at 100 yards everytime with open sights your set.