Not sure about the twist rate but glad to see you found something that shoots well for you. I have a smooth bore barrel with sights that I played with. It is easily capable of a clean kill out to about 75 yds with Federal's Tru-ball slugs. All the others I tried didn't shoot as well. I didn't try the saboted slugs because I figured it would just be a waste of $ with a smooth bore.
I did have my gsmith bore out the barrel and install a remchoke. I may get a rifled choke and see how it performs.
Good luck with your new toy.