rifled slug barrel
 

what's the best way to get what i believe to be the plastic residue from the slugs sabots from the bore?

A brass brush it will come out That is i youse but becouse it is ruff to get out i just do it at the end of the seasen.

Plastic residue
 

If ya want to buy something get some shooters choice and wet the bore real well. It'll come right out. If ya want to just use gasoline. Just be careful :D