I personally only shoot four shots. As that is probably what I would do at most in the heat of the hunt. My groups also aren't so good, which I think is more due to open sights than the rifle. Also due to the less stability of the "rest" ie standing with some support under my left hand. Any more excuses I can find ......

That group would certainly kill a buffalo or a deer. Do you know which shot was which and from which barrel. The four at the bottom look quite consistent ie are the two to the left from one barrel and the two to the right from another? Plus the higher one ?

Only five inches at 100 yards. You will have to shoot it at 100 yards and see how it does.