RS means rifle sights. If the barrel were rifled, that would be marked on the box and visible in the bore. Many barrels intended for slug use are smoothbore - they are for RIFLED SLUGS. Rifled barrels are used with SABOT SLUGS.
Magnum loads are OK. Steel shot should be OK.
With a 20" smoothbore barrel with factory rifle sight, I personally wouldn't be shooting slugs through my 28' VR barrel - but you can if you want to. It shouldn't hurt anything. I'd suggest an ImpCyl choke tube if you decide to do that though, slugs usually work better with no or slight choke.
A 20" RS smoothbore barrel is often used on a home defense shotgun, with whatever load the owner deems appropriate - birdshot, buckshot or slugs. They're often used for hunting deer with slugs, or for defensive use against bears and such, again with slugs - in this case, hard alloy swaged slugs like Brenneke or Dixie, and not soft lead Foster slugs.
If I were hunting with a 20in barrel and slugs, what would be the typical effective range?
That has MUCH more to do with the shooter than with the gun. How far out can you hit reliably with it? For me, a smoothbore barrel with rifled slugs is usually good for about 100 yards. For you... only your target will tell.
HINT - don't shoot slugs sitting at a bench like you would shoot a rifle, unless you want your fillings loosened. Shoot standing, using a vertical rest, like a post. Put something soft between the shotgun and the rest - your hand, or a towel. Otherwise the point of impact will shift away from the hard rest.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.