Hi Bob-

When you said that the balls at .760 are too big and that's cause they are bore diameter, did you mean to say that they are groove diameter?

What kind of rifling do these barrels have?
Is it the oval 'invisible' type or a bore and groove type of rifling with distinct lands?

Take a look over at the Lewis Drake website, there's a rather large bore muzzle loading double rifle for sale. The feature on it shows the bullets I speak of.
They're a conical type, 'driving band' type grease groove arrangement as I recall, and as-produced look as if they've been slugged through a barrel, they have pre-engraved rifling grooves set up to index with the barrel's rifling lands.

The gun I speak of is a Westley Richards 8 bore rifle.

The bullets.

The muzzles.

--Tinker