Cubb, a
Foster-style slug is simply a big hunk of pure soft lead with a big thin skirt, a hollowed base, swaged-in grooves along the sides, and a slightly hollowed point (in some versions). They were invented by a guy named Karl Foster, have been around since the early thirties, and are simply the second step up the shotgun slug evolutionary ladder ... the first being a pure lead round ball ... the 'ol "Punkin' Ball." As the pure lead ball proved to be a miserable performer in smoothbore shotgun barrels, Karl Foster set out to come up with a less miserable performer ... and that he did, with his new "rifled slug."
It was purposefully undersized, to be safely shot through a variety of shotgun bores of varying dimensions. It had a hollow base and a thin skirt that would, hopefully, expand to bore dimension, much like the Minie Ball of Civil War Rifle-Musket fame. It had a very weight-forward design, so that it would resist tumbling in the air ... much like a badminton birdie. And, most famously of all, it had those "rifled grooves" in the sides. Their function was not to impart a spin to the slug, but rather to add another safety dimension for those who might shoot the slugs out of choked shotguns ... the thin, soft lands of the slug easily swage down to bore dimension when fired. "Rifled" slugs do not spin when shot, but simply fly toward the intended target with that heavy center-of-mass keeping them from keyholing.
It is true that slugs can perform satisfactorily within their given limitations, which are many. In spite of their low velocity, poor sectional density, poor balistic coefficient, too-soft construction and marginal accuracy, many hunters drop deer with them every year. I'll hunt with a slug where I'm forced to by draconian political laws that inflict disadvantages on hunters because the powers-that-be don't know anything about firearms ... otherwise, give me a rifle any day.
As for those of you who are forced to use them:
KEEP ON SLUGGIN' 'EM!
By the way, Cubb ... the standard Winchester Super-X "rifled slug" is indeed a Foster style offering, intended for smoothbore barrels.