From Brenneke's website:
Question: Why do your slugs work with choked barrels? What do the ribs do?
Answer: In 1898 Wilhelm Brenneke invented the modern shotgun slug. The most revolutionary aspect of his invention were the ribs. They make it possible to shoot any of our lead slugs from any choked barrel, regardless of choke size. The ribs are compressed when they pass through the choke, and excess lead is squeezed into the gaps between the ribs. There is no danger to the shooter, to the gun or to bystanders, and no wear and tear on the gun. Excellent accuracy is retained. In 1992, we added minor improvements which resulted in even higher accuracy: five-shot groups of under 2" at 50 yards are possible. Depending upon the case length (2¾" or 3") you can effectively use our slugs up to 100 yard ranges. Note that our SuperSabot and our Super Magnum are designed only for use cylindrical full-rifled barrels, and must not be fired from any type of choked barrel.
Foster slugs are in the same boat, safe to fire from any constriction.
Opinions generally vary as to whether a cylinder or improved cylinder provides the best accuracy with slugs (just as there's debate over whether cyl or imp cyl gives the best buckshot patterns). Each gun is different, each barrel is different, and each shell is different.
