Based on Lancaster's representation, I visited the Brenneke web site and discovered the following:

"Brenneke-Flintenlaufgeschosse (mit Ausnahme der Geschosse „Gold“ und „SuperSabot“) sind für alle Arten von Chokes geeignet. Selbst ein enger Vollchoke stellt kein Problem dar. Die Ursache dafür liegt in den schräg gestellten Rippen, die nicht für Drall, sondern für einen problemlosen Chokedurchgang sorgen."

"Brenneke shotgun slugs (with the exception of the "Gold" and "Super-Sabot" projectiles) are suited for all types of chokes. Even a tight full choke presents no problems. The reason for this is the diagonally positioned ribs, which function not as a replacement for rifling twist, but assure a problem-free passage through the choke."

I have written to Brenneke directly and asked specifically about using their slugs in my pre-war drilling with 2 1/2" chambers. I will report when and if I receive a response.