RE: rifled slugs in rifled barrels
YOU CAN SAFELY USE plain lead rifled slugs in your rifled barrel, but these tend to lead up the bore sometimes, causing accuracy to drop off after a few shots. Also, leading is sometimes difficult to remove. I recommend using slugs like the Winchester Partition Gold, rather than Foster-type, or other soft lead slugs, if you have a rifled bore. Granted, these lead loads are a lot cheaper, but why bother with a rifled bore if you are not going to use the proper ammo in it??
Keep yore powder dry!!