Brenneke slugs are designed to crush fit. Lot of people think the "ribs" on those slugs are for making the slug spin without rifling in the barrel. While they do help in making it spin a bit for stabilization, those ribs are designed to collapse and fit throughout the firing. It's been ages since I shotgun hunted with a smoothbore (way back when I was a little critter) so I'm a bit out of the loop but that is the best smooth bore option I could track down for you. Good luck.