RICKT-
As you have already found, the old style Brenneke slug is a fine killer. Though not designed for a rifled barrel, it probably worked as well as it did due to the Brenneke slug being cast a little harder then the usual Forster (american) style slug, and way the cushin and base wadd are attached to the slug with a screw, so the whole thing goes down range as 1 unit. I bet that wadd may have engaged the rifiling, gave it some spin. Shot placement, even with a slug, still matters. Deer shot with a one ounce lead slug at 1400 feet per second in the chest just don't run away and recover.
Larry