RE: Rifled slugs in the smooth bore?
Shooting rifled slugs in a rifled barrel will not harm the barrel. It may lead it up a bit making cleaning difficult though. A rifled barrel does not improve the accuracy of rifled slugs so the rifling benefit is wasted. Another reason for the warning is that they make more profit on sabot slugs which DO get improved accuracy from a rifled barrel.
At 50 yards, a smooth bore with rifle sights, shooting plain old rifled slugs is more than accurate enough. Far more accurate than most people think. I have two 870's and I opt for the smooth bore partly because I am not willing to spend the money on the sabots. I may only shoot one round out hunting but I shoot at least 20 at the range. My last two shots in the field have each killed deer. Both were about 65 yard quartering shots, one towards me and the other away, both offhand. I hit both deer within an inch or two of where I was aiming so I have confidence in the gun and slug combination. I think that confidence is a big factor in accuracy, I do not second guess myself as I pull the trigger. (My range shooting also includes offhand practice.)
Rifle sights (or scope) on the barrel ARE important though. A bird bead may seem accurate at the range but that accuracy seems to fly away in the field when you are not aiming at paper.
Bob