From a rifled slug barrel, it may depend upon the twist rate you have to find the right load. What twist rate is your rifled barrel?
You mentioned slugs...but are you referring to slugs only from the rifled barrel, or rather any sabots/slugs in general?
What are your shot distances limited to?
If you have a rifled barrel, and you want to use a full diameter projectile, I have heard nothing but good news from Lightfield loads. Some full-diameter slugs from them (requiring a rifled barrel) are the Commander IDS, Hybrid Elites, or Commander IDS Plus. Those all use a partial sabot, but they have a full-diameter lead frontal portion with a locked-on rear stabilizing wad. I use the IDS in a bolt-action 12ga. When I perform at my best, I get true rifle accuracy: 1MOA. The recoil is a little more than average loads, keep in mind.
If you are trying to reach out further and want to use sabots, you may need to spend some money. As in, there are a lot of available sabots out there you may want to test out, and the 5 round box prices aren't exactly cheap. Again, with these, it's going to matter what twist rate you have.
The porting may help a little, but, the pressures that shotguns operate at are not they great, so I wonder if the effect of a port would work as well as it would on a high-pressure rifle.
