I dunno. I've always used bronze, and never had any problems... but in the back of my mind, I always wondered if, when using solvents to remove metal fouling, it was reacting with the brush? For a while, back in the '80s I think, somebody was producing a stainless bore brush. Used one for just a few times, but decided I didn't like the thought of using something that might be harder than the bore to clean it.
I do use one of those 'tornado' brushes when I actually use a rod to clean a shotgun, but I usually just run a bore snake through.
I do use one of those 'tornado' brushes when I actually use a rod to clean a shotgun, but I usually just run a bore snake through.