ya ill keep it legal...thats why i want the 18...i dont plan on using slug really...just some buck or birdshot...once i find which i like after testing at the range...but i mite get a 20 ill have to check them out...but with the 3 inch choke i can use both 2 3/4 inch shells and the 3 inch shell rite?
Couple things.......
The 20" smoothbore "slug" barrel works great for buckshot too. It simply gives you sights which gives more flexibility to use slugs. You can get it with either the fixed improved cylinder choke or with the Remchoke system which gives you even more flexibility.
If the receiver of your 870 is magnum capable and the chamber (not choke) of your barrel is 3" you can use either 2.75" or 3" shells.
The choke is the constriction of the bore at the muzzle, not the chamber of the gun.
And yes, as others have already indicated, changing barrels is about as easy as changing your underwear.