So I searched the forums and wrote a few emails and I'm finding out that it seems you can't cast a Brenneke style slug. It must be true because I sure can't find anyone who sells such a mold.
I have a Browning Auto-5 Magnum with a Hastings rifled barrel and it shoots Brenneke slugs like guided missiles. Unfortunately they cost about the same as a guided missile. You'd think they were made by Raytheon.
Is no one making, if not a conventional mold for these things, how about an unconventional one? A three piece mold maybe?
I know that they can be swaged but at a glance it looks like I'd need a budget akin to NASA to make this slug. That's out...I'm still having dry heaves over last weeks stock market ride...which has yet to end.