If you really do intend to make a brass gun barrel, and then actually fire it, You might want to do a little research.
There are about 500 copper base alloys called variously brass, bronze or copper. These names are a couple thousand years old and we have come up with just a few new alloys Hector bit the dust.
One problem you may have is the eternal conflict between machinability and a safe barrel metal.
Various Ampco grades, if you can find them, are strong and TOUGH but they might not machine so nice. One in particular, Ampco 18 aluminum bronze is made fror things like gears, or screw down nuts in steel mill service. One of my associates found some at the scrap dealer & asked me if it would suit for a small cannon. Looked up the properties and it seemed better than the 90% copper 10% tin alloy used for cast cannon barrels, such as the Napoleon's shot at my Great-grandfather.
The Clinton River Muzzle-loader who made the gun is an outstanding, and retired, GM toolmaker. He might have some opinions on the machinability. I suspect some of the 10% aluminum in this alloy may form a little aluminum oxide to wear out your tools. Oh, well - live with it.
What you may NOT be able to live with is the extreme lack of cross-grain ductility (toughness) in anyone's leaded grade. You may find leaded whatever strongly suggested by whoever has to machine this thing. Lead is a good lubricant, helps make nice short crumbly chips. You do NOT want a crumbly gun barrel. Them there nasty l-o-o-o-ng curly chips mean a good tough metal which should withstand reasonable charges.
The only reason I know to make a brass barrel is that it is pretty. Considering that shooting any flintlock is not all that practical, I guess a brass barrel would fit right in here.