I've never seen a rifled-bore Marinecote (nickel-plate finish) barrel, now that's not saying that they didn't make one... the easiest way to tell if you have a rifled-bore or a smooth bore is to take off your barrel and hold it up to a light and look down the barrel. If you see the lands & grooves, it's rifled, if not it's a smooth bore...
If it's a smooth bore you should use "Foster" style (rifled) slugs, if it's a rifled barrel you should use sabot-style.
You CAN shoot sabot's in a smooth bore but accuracy will suffer.
Smitty