My understanding is that copper removing fluid will as happily eat the copper alloyed in a bronze/brass brush as it will that in your bore. Hence the need for a nylon brush when using it.
I haven't found that to be much of a problem, but yes bronze brushes will be attacked by copper solvents. As I suppose, infinitesimally, will be the brass fittings on them, and the rod. The extremely strong copper solvents, such as Sweets will obviously be far more aggressive than things like Butch's, so probably not best for routine use.
Callum says that "
This task should be repeated if the patches are still coming out blue or copper coloured. It’s obvious that the copper solvents are going to react with the copper in the bronze brush and leave small traces of blue colouring but this is nothing in comparison with bullet jacket fouling."
Nylon brushes, if made without e.g. a brass core or fitting, won't, but they seem rather soft, so I don't think will give the same sort of scouring action as the stiffer bristles of a bronze brush. Using a brush (nylon or bronze) with say a steel core introduces other risks.
Just as using jags not made of brass, but harder metals, just might be a worry, e.g. when being pulled out after the patch has been taken off.
BTW, I clean my brushes after use, just a shake around in hot soapy water in a bottle, to get out any abrasive trapped grit/powder residues, and to remove any copper solvent that might slowly eat away at them afterwards. And of course wipe over the rod before and after use.
Perhaps there are good stiff nylon ones, but they must surely use thicker bristles than a bronze one has, and so may not be quite as effective. For the copper solvent to work fully I think that the scouring action helps, to get through any remaining layer of carbon still covering the copper deposits. BTW carbon is a misnomer, the black stuff is more of a cocktail of powder residues, baked on and compressed in by subsequent shots, and far more resistant than pure carbon would be. For some rifles that don't copper-up too badly, carbon fouling is the worst problem, and you really need to keep on top of it. And no, a boresnake isn't going to do much if any useful work, particularly a grubby one. I'd much rather pull through an ordinary bronze brush attached to a flexible plastic coated steel cable, as an expedient field kit, as several people make.
I understand that some modern cleaners, aqueous based, using cyanides, don't use the same reaction that leaves blue colouring, different colours being the result, and maybe less easy to interpret. The cyanide concentration is generally safe, you'd probably ingest more from a shot of e.g. slivovitz
On that point, whatever you use, do wear gloves and eye protection, it only takes the tiniest splash or spray off a brush to get in your eye to damage it, and certainly the traditional potions penetrate straight through the skin, possibly leading to long term skin problems.
