Try Gunbroker.com,or Guns america.Both may be found on the web.another source could be Numrich gun parts corp.They may be found on the web also.
No
At this time no,there are not any available.You could have a model 1300 slug barrel redone and have the gas ports added to this barrel.Then it will work fine in your model 1400 Winchester shotgun.
I have one that I would sell.
100-300 USD depending on condition
You can try Hasting's. They make after market barrels.
As long as the slug barrel is smooth bored(does not have rifling) then it can be used with bird shot.
Not anymore but I hear you can take a 1300 barrel and have gas ports added and that should work.
Yes. Slug barrels have a different "Choke" then shot barrels that make them more accurate when firing slugs, but you can fire slugs with a stock 1200 barrel.
Yes
Yes, I don't think you should have a problem, but your pattern may not be consistent.
Try e-gunparts.com, MidWest Gun Works, Shotgun News, gun show, gun shop.
According to Linda in the customer service department at Browning Winchester at (800) 945-5237 the 12 Ga models have a 1 in 28 twist rate.