3/8 in 25" is about 1:66, which is definitely a round ball twist.

Ballistic Products sells a ball, about .730 I think, that fits into a 12 ga. shot cup. I would definitely check their web site.

If you want to load an unpatched ball, you need a ball no smaller than .740, and .741 is probably better. Smaller, and the hot gas will melt the edges of the ball between the ball and barrel, causing poor accuracy and bore leading.