A nylon brush is used to apply so called "copper" dissolving solvents, of which there are only 2 or 3 that actually work (Hoppes9, Ballistol and ClP are not the ones either.
The solvent when reacting with copper leaves a blue tint on a patch so using a bronze(copper) brush would give a false reading.
Some purists use only nylon so as to not "harm" the barrel which makes no sense as a bullet is going thousands of fps down the bore along with hot gas and particles of powder.
Oh, almost forget if you don't use a bore guide with your nylon brush you will also wreck the bore from the cleaning rod.
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Very limited usefulness. Use a bronze brush if you're serious about cleaning. If a nylon brush "works", you didn't need to use a brush at all or you have only a slight trace of fouling, whether it's copper, carbon, or both.
I use nylon brushes to apply & scrub Copper Solvent (Shooter's Choice Copper Solvent work well), a different brush and rod are used for rubbing alcohol to neutralize any solvent, then a different Bronze brush for powder solvent & scrubbing. Copper build up and powder varnish are in alternating layers that REQUIRE different solvents. I have bought used rifles that had 30 layers to be removed.
Cast and lead bullets will shoot OK over copper fouling. But jacketed bullets shoot horrible over lead build up!!!
I always believed they worked well for light cleaning. But I always prefer a bronze brush, and avoid the stainless steel ones. While it is true a copper solvent such as Shooter's Choice could destroy a bronze brush, cleaning the brush with a cleaner/degreaser after use could save the brush.
Not normally very effective . However, those of us who own vintage factory nickel revolvers, use the nylon on delicate old nickel. But they are more like scrubbing the bore with mild solvents..