It is very much possible to wear rifling prematurely with excessive brushing.
Stick to the bronze brushes, don't use stainless (too aggressive), and don't keep on brushing till the patches come out perfectly clean.
On a newly acquired Mosin, I've used both Shooter's Choice Copper Remover and JB's Bore Paste to get rid of most of the previous copper deposits that may be there.
The copper remover will get out the copper, obviously.
If you use that liquid, or another brand, it is NOT necessary to use it EVERY time you clean, and not advisable. Some can be harsh in the bore.
Don't use it in conjunction with bronze brushes; think about it- it's a copper dissolver & what's in bronze?
Copper removal liquids are best used with patches, keep "patching" the bore with the liquid till it no longer comes out in any shade of blue or green.
At that point- stop.
Run an oiled patch down the bore, followed by a dry patch.
The dedicated copper-cleaning solutions are not regular cleaning fluids & should not be used regularly, everytime you clean.
Once to start out on a "new" Mosin, and occasionally after that, with high-volume use.
You can go hundreds of rounds without using it again, unless you have a particularly rough bore that's building up deposits.
Heavy copper deposits can frequently be seen with a good bore light.
Re-use the copper cleaner accordingly.
Remember that some are based on ammonia, which can be hard on the steel.
The JB's Paste comes in little plastic "tubs".
It is applied THINLY on patches. Write that down- THINLY.
It'll bring out a surprising amount of gunk from rifling, even after you think you've got it clean by conventional cleaning solutions.
You follow your standard protocol with normal cleaning however you do it (the boiling water trick, the Windex patches, Siberian Yak Urine, whatever), then run three or four patches down the bore with the paste (follow instructions), then clean with solvent & patches.
Don't overdo it. The paste is a very mild abrasive & it'll wear your rifling if you abuse it.
Again- NOT EVERY TIME YOU CLEAN YOUR BORE.
These are both proven methods that can get your barrel pretty much down to bare metal.
Another thing to try, if you're losing sleep over it, is to run a couple solvent patches down the bore three or four days after you've cleaned the barrel.
Gunk can float up over a 24-hour period after cleaning.
Just DON'T over-use any metallic brush.
Denis