This is what I have gotten in reply from a Brenneke USA rep:
I will see if I can answer your questions comparing our 3 inch Black Magic 1 3/8 oz with our new 1 1/2 oz Magnum Crush. It is somewhat confusing so bare with me.
Brenneke USA has in the past offered two 1 3/8 oz 3 inch slugs - - the Black Magic and the Gold Magnum. Both slugs have the same ballistics. However, the Gold Magnum was designed for rifle barrels only and the Black Magic was designed for both smooth or rifle barrels and any choke.
First I have to point out the purpose for the ribs on Brenneke slugs. The ribs are there only to allow the slug to pass through the constricted areas, the forcing cone in front of the chamber, the inside bore, and the choke. As the slug passes through these cionstriucted areas the ribs compress and keep the slug in its original shape for better down range accuracy and penetration. With the Black Magic, the ribs go in the traditional direction and can be used in either a rifle or smooth barrel and any choke - - improved cylinder, modified, or full choke. However, the more open the choke the better the accuracy. Competitive slugs are not recommended by them to be used in a smooth barrel with any choke other then improved cylinder.
The Gold Magnum was designed for rifle barrels only. The ribs on the outside of the Gold Magnum go in the opposite direction to grasp the rifling for somewhat better accuracy. The ribs on the Gold Magnum are more rigid and do not completely compress as the traditional ribs do on our other slugs. Therefore, it is necessary to have only a cylinder choke or open choke with no constriction. Most rifle barrels do not have any choke. This is the reason the Gold Magnum is recommended for rifle barrels only. If you have a smooth barrel shotgun you could use the Gold Magnum if it has an open choke which is unusual for a smooth barrel. You can see how difficult it is to explain this to the average hunter - - so Brenneke chooses the best way as "for rifle barrels only".
Now back to the new 3 inch, 1 1/2 oz Magnum Crush and your question. Brenneke designed the new Magnum Crush, a very hard hitting slug with both deep penetration and accuracy using the same opposite direction ribs as the Gold Magnum. There was no reason to continue with the Gold Magnum and the Magnum Crush replaces it. If you have an open choke with no constriction you can use it in your smooth barrel. Without the rifling in the barrel it may not be as accurate? I am attaching one of our current catalogs and on page 15 you can read about the ribs on the Brenneke slugs and their purpose as I have pointed out above.
I am not familiar with Brenneke having a frangible slug. If you want to let me know which slug you are referring to maybe I can somehow help you?
Hope this helps - -
Charlie Hays, Brenneke USA
EDIT: The frangible slugs are manufactured by Ddupleks not Brenneke... haha my bad fellas! Here goes another email!