Scott,

As far as I know nobody manufactures a 4 bore barrel that is why any that i have read about have had to make their own.

However if you were to contact one of the barrel makers I'm sure they would be willing to make you one for a price.

As I said in an earlier post a friend of mine is in process of building a barrel making machine, once Bob is up and rolling I'm sure he would be happy to turn one out for you.

Gregg