The only thing I would trust that I have seen on this is an article by Dr Arne Hoff in "Arms and Armour Annual Vol 1"
He analyses two similar barrels by Werder, and the Belgian gunsmith Selier. Werder's (cold hammered) has a wall thickness at the muzzle of 0.60mm (!!!) and Seliers 2.00mm. While Hoff calls the material "brass" in the article he states:
"Both brasses were made of a brass containing a small quantity of lead; there was a difference in the percentage of copper in the alloy...the brass of the Werder barrel had a little more tin, iron and nickle than the Selier..."
There is no mention of zinc so really hes talking about bronze. He states that the Selier barrel was cast as a plate, which was bent into a tube and then soldered (more likely brazed) shut with reinforcing pieces across the joint. The Werder was cast as a hollow tube, then cold-forgeded and machined inside and out.
Later he says aproximately 1/3 of all 14th-15th century barrels were cast from copper alloys (ie bronzes and brasses):
"it is difficult to establish how many surviving specimens are true bronze (copper and tin) and how many are made of brass(about 80% copper, 20% zinc)... but a superficial estimate based on surface colour points to an overwhelming majority of bronze... In the 16th century when brass production was increasing, the opposite seems to have become the case, and in the following centuries bronze barrels on handguns diminished almost to extinction"
he has a few other things to say, mainly that the majority of brass/bronze barrels were cast as hollow tubes, or bent from plate and soldered, but he soes quote some examples of dsmall bore pistols and long guns that might bave been drilled from a solid bar (he says the bores are almost always slightly off centre)
Anyway, hope this helps...
Bronze, BTW has excellent anti-corrosive properties. I think the alloy called Navy-G bronze is the same as the "gunmetal" the board of ordnance used to cast cannon and mortars with at the Greenwich foundary.