if it was me i would get a rifled barrel new ones go for about 160 bucks and get some good sabot slugs like remmington copper solids or similar i went thru the same issue back when we couldn't use a rifle in new york state all i had at the time was a browning a-5 smooth bore and had to use those rifled slugs which at the time sucked maybe better now but a rifled barrel and some good sabots are the ticket in my opinion. good luck !!
If you are going to stick with that gun then this advice is the best option. A rifled barrel and sabots are a huge improvement over a smooth bore and rifled slugs.