Like Gumpy's post: "Synthetic brushes are good for rubbing solvents onto the bore to spread them to allow them to work before cleaning with a bronze brush before patching out
Gumpy "
Unlike Gumpy, I use no bronze brushes any more. Not needed and, well, I'll explain why.
I use synthetic brushes for introducing solvents into the bore of ALL my rifles, from .17 cal. to my 12 bores. I do not use bronze brushes now at all. About 50 years ago, I found a good solvent does the cleaning for me, scrubbing back and forth with a bronze brush just wore out the brushes over time.
I found a steel cored, nylon brush introduced good solvents into the bore, without the solvents dissolving the screw that captured their bristles, unlike the twisted brass or bronze screw that holds the bronze bristles. Scrubbing bores with a bronze bristled brush did nothing to clean help them is seemed, but did introduce more copper/bronze into the bore in the form of non-fouling liquid, staining the patches blue that said solvents dissolved from the brushes as well as from the bore, EVEN WHEN there was no guilding metal fouling left in the bore. I'd read about that in Precision Shooting mags. and tested it with a perfectly clean barrel. A bronze bristle brush run back and forth in a perfectly clean PacNor barrel left blue on the wiping patch even though there was no copper or guilding metal fouling in the bore. It had no fouling at all, however, from the patch, it appeared there was still metal fouling in the bore. NOPE!
Well, it's a mostly free country, use whatever you want. I'll continue to use nylon brushes to introduce a good solvent into the bore. I've found I really like Butches Bore Shine as well as the foaming Wipe Out. It is used with an overnight soak and 1 patch in the morning takes out ALL of the guilding metal or copper fouling.