Slugs can cause the soldering on double barrels to fail.
Rifles slugs are for smooth bore; sabot slugs for rifled barrels.
These little shotguns are still a fun novelty, but not the best choice for defense from wildlife.
I would love one of these slightly longer, like 24"/
I've got mine in and the manual says exactly:
If the barrel is marked 3 inch it will accept 3 in & 2 3/4 inch shotshells. We do not recommend using slugs in this O&U shotgun, except for cylinder and improved cylinder choke tubes.
These are cylinder choke. No need for people getting worked up, shoot slug until the cows come home.
*in Homer Simpson voice*
mmm... slugs...