You got it wrong...
A smooth bore Rem 870 slug barrel would have an Improved Cylinder fixed choke. It is designed to allow rifled slugs to pass thru the choke with minimum deformation of the rifling which is machined into the soft lead bearing surface of the slug, itself.
A shotgun barrel with rifling inside it has no choke designation at all, because it is designed to impart spin to a sabot slug, the same way that rifling inside a rifle barrel does. You never hear about a rifle having a choke in the barrel, do you? Neither does a rifled shotgun barrel.
The rifling inside a shotgun slug barrel engages the bearing surface of a plastic sabot, and never touches the slug, which is encased inside the sabot sleeve. The sabot sleeve is in 2 parts, which separate and fall away from the spinning slug encased within, as the spinning assembly exits the muzzle and goes downrange.
A sabot slug is not the same as a rifled slug. Two different animals, entirely.
A rifled barrel is not the same as a smooth bore barrel. Again, two different animals, entirely. Different designs, different performance.
"Deer barrel, Improved Cylinder, rifle sights" means smooth bore, fixed Improved Cylinder choke, for use with Foster/Brenneke Classic, and/or Federal Truball rifled slugs. Not for use with sabot slugs, and no rifling in the barrel.
Rifle sights for the Rem 870 slug barrel described are rather large, and pretty useless beyond about 50 yards distance, IMHO, because the front sight blade covers up half the length of a deers body beyond that distance. That prevents precise aiming, as far as I am concerned. So, I scoped mine, in 1974, when I bought it.
Currently, "deer barrels" are available with a cantilevered scope base attached to the barrel. I believe most of these barrels have rifling inside the bore, and no choke designation. They shoot sabot slugs, only, and are more accurate at greater ranges than the barrel I have (which I believe is the same smooth bore, rifle sights barrel you saw advertised).
