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he Browning Slug Barrel is smoothbore until the last 2-3" then chokes down to .715 or IC choke. Ithaca did research with their Deerslayer M37's and found a little choke helped to center the slug along the center line of the barrel as it exited. I think Browning followed that idea with their barrels. Both the Browning Slug barrel and the Hastings Rifled barrel are 24" with rifle sights.
From what I read, guns shooting lead projectiles often will be more accurate when having a slightly choked/tapered rifling at the muzzle. Some of most accurate modern airguns are smoothbore with only about four inches of rifling at the muzzle. It probably has to do with lead being non-elastic - once swaged down passing a tight spot in the barrel the boolit is no longer fully supported by the rifling. The same thing would happen to a lead slug in a smoothbore which is why I believe slugs with an attached wad is easier to get to shoot well; the wad keeps the slug from wobbling in the barrel.