I suspect you won't be able to find a longer barrel for a gun with the longer defender mag tube, and if you did it'd probably cost almost what you'd expect to pay for a neat used gun at pawn shop or gun show.
The stock barrel oughta be a cylinder bore, what exactly are you looking for ?
Lotta guys berate the 1200/1300 winchesters because of the alloy receiver, I've got two of them a 12 and a 20, the 12 is over 25 years old still looks and works just fine. That alloy comes in handy if you ever plan to lug it around all day long hunting birds or bunnys
You're right, the stock bbl is a cylinder bore, I was tossing around the idea of having a barrel for clays or skeet. But after looking around... yeah you're right again, the extra bbl goes for as much as good used shotgun ($205 plus shipping).