like AA said you may as well just use bird barrel if you want a smooth bore slug barrel. They do make riffled chokes specifically for that but usually a IC choke works well.
you understand why that happens right....??
Did you try different kinds ? Usually tumbling is caused by the bullet not spinning fast enough. The heavier the hunk of lead the faster its gotta spin.
Not to sound cheap but why shoot a sabot for 2 or three bucks a round when the rifled slugs work just fine. Who knows though.