Hanshi, for your information the bare ball load that I use and is recommended by some who shoot in international competition, uses 2F or courser powder, usually about a hundred grains, no nitro, or cushion wads of any kind, and just hornets nest, or tow, to hold the ball in place. Adding wads, and nitro cards, interferes with the cone of gas that centers the ball.
I think tighter patterns are common in smaller bore smoothbores, but the string is longer, which mean your lead will need to be increased.
I have not heard that smaller smoothbores shoot round balls more accurately than bigger ones. They should shoot flatter than a larger, heavier ball, but probably will lose their velocity quicker being lighter.
Hungry Horse