An oval bore is really a barrel with very wide rifling grooves so presumably you would use the same dimension bullets as for a conventionally rifled gun of the same dimensions.
Having said that however, I have to confess when I made a mold for my Lancaster Colindian, I made the slug 1/2 way between bore and groove diameter. I have shot it a bit but not enough to decide if I could get better accuracy from a groove diameter slug.
The maximum length of the slug can be determined by the Greenhall forumla; 150 diam squared over twist = length. If you reverse length and twist you can calulate the minimum twist to stabilize a given bullet.
cheers Doug
