I was wondering what the story was with rifled chokes...

I have a Remington 870 w/28" smooth barrel. I know you should shoot rifled slugs through a smooth bore, and sabots through a rifled barrel. If you stick a rifled choke on the end, what should you be using then?

Additionally, how much accuracy does the choke offer? If a smoothbore is at a "0" for accuracy, and a rifled barrel is at a "10" for accuracy, where would you think the choked smoothbore would land? Is it worth $40 of accuracy?

Any insight would be greatly appreciated. Thanks ahead of time!