You could try riffles chokes 1st, that "MIGHT" give you an indication.

But a better option would be insert barrels that you can index and adjust poi.
That would limit you to smaller calibres, but trying to sleeve two shotgun barrels and get them to shoot accurately is an expensive flip of a coin.

My money is on turning it into a ball and shot gun, my factory regulated one hits slugs from both barrels on a 10" gong at 100m just using a flip up sight and a shooting stick.
It could hit a smaller gong but with those sights and my eyes the 10" gong is as small as I can see.

I've got magtech brass cases that I load with 32gm slugs that are easy to load and hit hard, although I haven't had the opportunity to shoot any game with them past 40-50m so far, and in truth I prefer RB and hunting game out to 50m anyway.