Bronze is just the name of PMC's line of ammo. They have gold and silver as well, but don't use gold or silver in thier ammo. They are still brass casings, with copper-jacketed lead cores.
I don't see why a ammo company would use bronze. It would be more expensive to manufacture. But it would probably be fine for the firearm. It's a soft metal, we use bronze brushes, don't we?