Well lets see how i can explain this.
Whats harder lead or copper compared to steel ? Your wondering where im going with this. If lead or copper can take the rifeling out of a steel barrel that should show you that the hardbess off the metal doesnt affect it like you explain. Yes the brush will wear out faster. It will still remove particles. On a molecular level ... You might loose two part brass to the one part nickel. Or a grinding disk. It not made of a peticular metal thats harder than the one your grindin. It the texture of the surface. Even copper being soft. When you use a steel fitting brush on it the steel wears down. That goes to show that a soft metal can wear a harder material. I have been wrong before ... But usually when someone says i can do something ... Watch and see

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Think of it like this. Would you ram a brass brush up and down your steel barrel in a precision rifle for two hrs going through 10 brushes ? I think you see where this is going ...