Rifled slugs are usually available in boxes of five. Buy a box and see how your gun shoots " as is" .....unless you have some serious flaw, you can shoot rifled slugs with the equipment you currently have.

I have a smoothbore shotgun (barrel sawed off to 19" , iron sights added, that shoots well inside of a grapefruit at 40 yards. (cylinder bore.)

You might want to consider that with your current barrel and buy a second barrel with screw in chokes for next year' s upland bird season.