You can’t bugger up a steel barrel with a brass brush any more than you can scratch a diamond with a shard of glass - a bore brush for an air gun is however not a useful cleaning method. You are not trying to remove powder residue ‘cos there isn’t any.
I’m a huge fan of polishing barrels though.
What you are doing is removing micro burrs from the rifling. Not one of my polished barrels has ever been pellet fussy, none have needed regular cleaning and none need ‘leading in’ to shoot well.
If you think about it on a microscopic scale it makes sense. A rough edge on a land will affect the projectile. Leading in is just smoothing out that rough edge by filling it with lead scraped off pellets to make it smooth. Smooth out the steel then there is no need - also smooth steel doesn’t pick up lead in the first place. Just basic mechanical engineering. Tribological wear will mean that some lead is deposited, but orders of magnitude less than if (under a microscope) your barrel looks like a saw blade.
I have tested this by mounting a pellet in a drill bit and running it against a smooth knife blade - essentially zero lead transfer.
Soft material against hard and smooth material- it is blindingly obvious.
