Like the others said, some of them shoot great with nearly any ammo, some shoot great with only certain select ammo(not always the expensive stuff either), and some only shoot good enough with "any" ammo to barely hit a pop can at 30+ yards no matter how hard you try.
With that said, I've owned at least a dozen or more of them, and shot a whole bunch more over the years, and on average, they'll be at worst a 2" at 25 yards gun with "some" kind of ammo. Most I've shot would do 1.5" or better at 25 yards, and some(like Ken's) are as accurate as a high dollar target gun. My current 5.5" stainless will shoot the cheap 36g Federal bulk HP's from Wal-Mart into 1" or less at 25 yards very consistently off a good rest. It'll shoot a few other high priced loads into groups that look like a rifle shot them, but honestly, if I can hit a squirrel head at 25 yards, I'm tickled to death, so I'm not wasting my money shooting up high priced ammo in it. These Federal Wal-Mart specials are less than $10 for 550 rounds, and perform on small game very well to boot.
If I was you, I'd pick up a box of these Federals right off. Just about every Single Six I've ever seen shoots this ammo as good or better than any other, and in most of them, it's more than good enough for any small game hunting you'll be doing. Make sure to get the 36g HP 550 bulk packs because some of the other low priced Federal loads are no where near as good.