I don't know about all the makes, but you can buy a Remington 870 that will accept both a 20" and a 28" barrel, and if you like the looks of a magazine as long as the 20" add a 3 shot magazine extension.
http://www.ableammo.com/catalog/remi...-p-126101.html
Check the barrels available from Remingon as for the 20". I assume you want a smooth bore that takes chomes and as far as I know the factory only offers one with rifle sigth. Others may offer something else for a Remington. I dunno but Mossberg even makes some barrels to use with an 870.