Hi OlPainless ...................... OldYeager here
Interesting topic here. I have wondered about that question too, for a long time. I always assumed the slanted 'ribs' on the slug was to impart a spin on the slug once it left the barrel. Air striking the slanted 'Fins' would seem to caust the slug to start a spin. I could be wrong. Never have seen any data on such. Maybe the stability is a combination of all the mentioned factors. You know, weight forward-hollow rear, 'Fins' etc.
Had a smoothbore Mossberg 500 with Slugster barrel once for home defence. Slugs and Buckshot, it was a shoulder stomper. Gave it to my son in Iowa. He decided to use it for his annual deer hunt in Iowa and went to the range for some practice. He came back from the range, put the Slug gun back into his gun cabinet. He said his shoulder and upper arm turned purple with bruises. Said no thanks, but he would continue to deer hunt with his Remington 700 in 308 cal.
As a young man I grew up using a 410 SS. My cousin and I would take it down into the cane breaks and log jams near the Mississippi river. We 'hunted' Cottonmouth snakes with #6 birdshot, but we used rifled slugs for the long shots on big loggerhead turtles Sunning on logs sticking out of the bayous. We discovered that the slugs went pretty straight, but did have a pronounced drop that required 'hold-over'. After a few rounds downrange you could easily calculate the holdover required and bust those big turtles.
But I guess these days, busting snakes and turtles wouldn't be PC. I got a daughter-in-law that has forbidden me to tell my grandsons these stories of old where we actually Killed things.