0000 Buck is nominally .375" and each ball weighs about 80 grains. 6 make a 1 1/8 ounce load and 8 make 1 1/2 ounce.
I have a .375 round ball mold I got for making plinker loads for my .375 H&H.
I load 6 or 8 balls in 12 gauge hulls with data appropriate for the payload weight and they shoot very well from a cylinder bore barrel. The more choke, the worse the pattern.
I think the reasons you don't often see them loaded commercially is the reduced pellet count vs the usual 9x00 Buck, and the increased recoil in the heavier loads required to get more pellets.
They are fearsome loads, though. They would be just the ticket for hogs, as I have used them, and for deer or bear, which I have not.
They would also be very effective on human targets, the rough equivalent of being hit with 6 or 8 light 9mm bullets at 1200-1400 fps.
Another great use of the .375 ball is to run it through a .358 or .360 sizing die and load in any 35 caliber case for super light plinking and small game loads. Works great in .38 Specials.