Good round ball loads do very well in my experience. I have shot few other slugs that do as well to 50 yards or a little further. Much beyond 50 or maybe 60 yards though groups do tend to open up. I have never tested mine in 10 yard increments to 100 yards though so can't say just how far they are consistent to.
Out to 50 yards I can usually depend on 3" to 4" groups with both round balls in shotcups or naked 0.735" ball on a hard card wad column.
I can say though that the 0.735" round balls do tend to roll into the bore because I have recovered several with skewed "belts" around their equators. A donut wad, inverted gas seal or a scoop of COW should help avoid that... as would 3" hulls in a 3" chamber I am guessing.
In general I lean towards a 0.662" or 0.678" round ball in a shotcup as they are easy to load and shoot very well. The shotcup of course eliminates any chance of rolling in the bore.
After my last range session shooting 50 slug loads from the bench I flinch even thinking about shooting them prone. Yikes!
Longbow