Sorry for my implied flipant responses earlier. Just that I've spent a # of $s looking at this "thing." Really wish I hadn't as it's cost me plenty in shooting up various slugs 'n stuff.
Chock up my responses as a "bad hair day."
I've tried BRI-types in smooth barrels & they do seem to be pretty darned accurate & who could argue with a .50 caliber, 1 oz slug at fairly hefty velocities? = not me, & if they pattern in yours = go in peace & shoot well. I've had excellent results w/'em, but can't find them any more. (sigh)
The sub-caliber rounds are purported to be the cat's a$$ regards trajectory, etc. from a rifled barrel, but then again, you're shooting a "rifle," no? I've had zip good results with 'em in smoothies and rifled chokes.
And, giving up everything a shotgun is supposed to be = an all-purposes/do everything kinda-gun ..... with any of the "extra" chokes installed.
Brennekes seem to pattern better than anything else I've tried - short-barreled/open chokes to 21"+/modifieds .... YMMV
Dedicated rifled barrel for sabots? I betcha you could approximate a .45-70 load, but you'd also lose eveything "shotgun-wise" regards versatitlity.
If I was gonna do that, I'd just use a rifle.
If I still wanted a shotgun, I'd go with a known & dedicated slug-round I'd proven out beforehand.
(yup. waxing philosophical ..... )