OK, lets say, for argument's sake a .50cal 500gr bronze point spitzer boat tail could be loaded and stabilized for a 200 yard shot, say @ 1900 fps.
At 200 yards, to make a clean and accurate shot on athe kill zone on a deer, a 9 inch circle... that's what scopes are for. Now why would I want to put a scope on a shotgun when I aleady have a rifle that can do that distance, with considerable less drop and recoil? I'd gather that a "long range" shotgun slug would likely be banned in "shotgun only" areas the same way sabots are illegal or blackpowder hunting.
As far as stabilizing fins, rifling provides all the stability you need for 1000 yard rifle shots. Finned projectiles create spin by having slightly angled fins, that reduces cavitation created by the stress of acceleration. QED You don't need fins UNLESS you have a sabot. Now at shotgun velocities, the rate of twist is pretty slow. By the time your slug hits its target say it fifty yards, its not spinning like a buzz saw, rather drowsily spinning on its own axis once every foot and a half or so. (FIGURES NOT EXACT... just to illustrate) That means at 50 yards, your rifled slug has spun 100 times, and can expect to hold a one and a half inch group.
Now to recreate that accuracy you must now create a projectile, that must stay exactly true to the axis of the bore (which the traditional slug does by being hollow based) while inside a sabot, inside a plastic cartridge.
Now the fins MUST me machined accurately enough to guarantee that spin effect of say 100 times at 50 yards. Well, if you've ever machined lead, you realize that the act of firing the shell will affect the shape of the slug, that's why saboted bullets are made of copper or harder alloys. Now copper isn't that strong and can be bent with your fingers, bronze is pretty brittle and harder stuff makes a bullet that will penetrate some armor btw, and may not be legal where you hunt.
Overall, I think the issue is one of reinventing a wheel, and a wheel that is of limited utility, high cost and hard to manufacture.
As far as being lethal. Rifles and shotguns kill by generating Shock, and penetrating vital organs. Broadhead arrows penetrate deeply but do not create enough energy to create secondary missles from broken bone, bullet fragments or "hydrostatic shock" that bullets do. The purpose of the broad head is to INCREASE the cutting surface exponentially and create a wound that won't close up, and allow an animal to bleed out or be tracked more easily. You'd be suprised how many hunting arrows shoot right through a deer or elk. BTW arrows spin to stay stable, that's what the fletching is for. The effect of a spring loaded wings or blades on a projectile fired from something as primitive as a hotgun.. I doubt you'd get consistant accuracy, again based on the need of the projectile to stay bullet shaped until it hit something, yet be tough enough to not spring open upon firing.
That's my 2 cents.
