Those bullets were cast with a pure lead nose of about 55 grains with the rest of the cavity quickly filled with WW + 2% Tin. I cast a few noses only and found that they have a convex base which will project into the harder alloy when the mould is filled. From the pictures and Vance's weights, I am guessing that the nose expanded violently, forcing the front of the harder body apart before disintegrating or separating. Since the harder body expanded to .410, I would guess that the nose reached .55 or .60 caliber before failing.
It's sweet because Vance and his father are my neighbors and good friends. It's sweeter yet because we got into this because a know-it-all told us that such bullets could not be made to work in microgroove rifling.