OK, I've been lurking on the topic of roundball guns.

Now I have a question:

I have an Ithaca 20-inch slug-barreled M87. It is a rare bird, actually, as the 87 monicker was only used for a few years. It is in fact a 37.

Anyway, the bore of this gun appears to be .716 with a bit of constriction at the muzzle, reading .712 or so.

I see Lyman makes a .715 ball mold, Lee a .690.

My question is, which to get? The Lee mold is cheaper, and I'll probably get that one just to start, but which one is mostly likely to shoot better?

Seems simple to assume the .715 ball would, but I am wondering if that ball would skid on the barrel wall, lead and go akimbo and or be effected by a wee bit of constriction at the bayonet end, where the .690 might stay straight due to being centered in the bore by a cupped wad {upside down shot wad base per Daryl's instructions for undersize balls}?

Daryl?