The cup wad that is cradling the ball, holds the it centred in the bore during it's trip up the bore due to pressure of accelleration - ie: G-force. This will also prevent the undersized ball (which causes no leading) from spinning in the bore which causes it to take on a wild flight before travelling a descent distance. The faster you drive the ball, the longer it's 'accuracy range' as the farther out it gets before taking on a spin and curving off the path of the sights. My own double would hold in about 8" at 100 meters, 12" offhand, which I considered it's maximum range for moose and about 75 yards for deer.
The ball is large enough and heavy enough, it doesn't need to be harder than WW metal, and indeed, pure lead can also be used on our ungulates.

As WW ball, about brinel 12, will bust both shoulders of a large Canadian moose, and stop underneath the hide off side. If only one shoulder is hit, or no shoulder, it is likely to exit. They do not have to be harder, which makes casting easier.

Jeff Tanner (UK) will make any size ball mould you want - for around $40.00 US shipped. My last one (recently) cost $38.00US.