I've had a hastings cantilever barrel for 10 years. Once I FINALLY took the time and shelled out the money to see what it liked....and took the time to REALLY get that sucker clean it'll shoot cloverleafs at 50yards all day long. If you consider a deer a 4" target that means it's a 300 yard gun. Nah, my point is that it's more than enough and that's about as good as I ever hear of non-fixed barrels doing.

I have a 870 in 12g

18" is no problem...actually may help with accuracy

You WILL need to buy sabot slugs to reap the benefits of a rifled barrel

You DON't need to pay the extra for 3" slugs...It's highway robbery as it is.

If you've still got good eyes and don't need the magnification look at a "reddot" scope instead of a normal scope. Bought one for my pistol and slapped here on my slug barrel for kicks...I just may buy another!!!! Low-light becomes a non issue and gun fit becomes a non issue. Find the little glowing red dot, when its in the right spot...pull the trigger! Eye doesn't have to be lined up at all.

I'd also suggest putting some swivel studs on your shotgun to attach a real sling to. Those "shotgun slings" are just a pain.