Bob - a friend from Alberta send me a package with a bunch of the 12 bore Lyman Sabot's slugs along with some WW red 'field' wads. The slugs fit the wads perfectly. The ones sent are cast from WW metal and weigh an average of 506gr. They are 14 bore in size which is .686" approx - slightly smaller than a 12 bore full choke muzzle's.690",ie: still a good size. Expansion isn't needed due to their diameter.

Since I don't have a rifled 12 bore barrel, their hollow base is required to help keep the nose forward. The 20 bore slug pictured looks a bit longer than the 12 bore, ratio-wise, diameter to length.