There are any number of us sitting here drooling over this barrel and what we could make with it. We do not want this thing turned into a Bubba cannon.
Here's my suggestion to you. Get your self a piece of 1018 steel. Get you self a set of cannon plans. Build a cannon and learn how to use you lathe and learn what a cannon looks. 4140 is not that hard to machine if you have the tools set up correctly.
Here is a plan that would make a good learning product.
I made my first lathe turned cannon from those plans and from 4140, thirty years ago.
Making the cannon will teach you basic turning, hole drilling, taper turning, some free hand profiling as will as milling practice for making trunnion pockets. Get a copy of South Bends
How to run a lathe It's available as a print on demand book
When you open that page if you click on Get this book in print in the upper left corner and a drop down menu will open. Click On Demand books and link to the Expresso book machine will open and you will find a list on demand printers.
While you are learning your lathe you can do some research and find a cannon to replicate
That reference the Jeff made to
[Modern Guns and Mortars Adopted by the United States Land Service is a good one. I already have a print on demand account with the University of Washington. I ordered the book and had it in 7 days for $10 and postage.
There some very basic rifled breech loaders in the book . The drawings are not dimensioned but they are scaled. It would be very easy to use a dial caliper and measure the drawings, put the dimensions in excel and scale then up to fit GAU barrel.
You could make a very nice cannon that you could be proud of if you just take the time to learn.