Ken- I didn't follow you post, you were keeping slugs within 10" with Cylinder bore or Buckshot? IME, rifled slugs from a smoothbore should be much tighter than that, and even with a full choke buckshot will spread more than that.
combat- slug accuracy from a smooth barrel has been much underrated by the gun press (maybe to sell rifled barrels?...nahhhh couldn't happen!). I used an old borrowed remington 870 express with a full choke 20"(?) barrel and Brenneke rifled slugs to take two deer last year, one of which was running at 70 yards, and I'm no Chuck Connors. I put my faith in the versatility of the smoothbore, if I need better than 5" groups at 100yards, I'll run away and look for a rifle