For starters it all depends on what you consider accurate and at what range. It would also depend on the particular shotgun. Some shoot pretty well with a smooth bore and rifled slugs, some not so much.

I don't know that you need a 26 inch barrel, I have seen smaller guns be pretty accurate with the right slugs. I would stick with a modified or improved choke though. Just try some different slugs and see what happens.

My mossberg with a smooth bore was good for about 60 or 70 yards with maybe 5 inch groups. I have buddy that has a Franchi (spelling?) lightweight 20 ga that will shoot rifled slugs into a 5 or 6 inch group at 90 yards with twin beeds on it.

If you are going to look for a shotgun with a particular barrel length or buy a barrel for it I suggest just getting a fully rifled barrel with decent open sights or a scope mount. You could probably get a H&R with a rifled barrel and open sights for what a barrel on most shotguns would cost. And with something like that you should be able to get 3 inch or better groups at 100 yards if you find a load it likes. However the saboted slugs are pretty expensive, like $10 and up for five rounds.

Paul