If you're using copper solvent then a bronze or brass brush will cause the solvent to turn blue from its own copper content, you then won't know if you are cleaning out the copper fouling or just dissolving your brush. That said, it is my own belief that if using a nylon brush to scrub the bore, you will wear out your elbow long before you have removed the fouling.
Also, the barrel will wear out the brush, not the other way around.