log man, you are right and wrong
I scratched the outside of my Glock barrel (the square receiver section) with a bronze bore brush, because who cares, it's just a handgun, and not the most accurate one in the world. Having said that, I love my model 23, and it is my first line of defense usually in everyday life.
I did scratch the outside of the barrel, and yes, there were fine COPPER scratches. I wet a patch with Hoppe's Bench Rest 9 copper solvent, and rubbed it in circles with my finger for about 15 seconds, and dried it with a cloth. The scratches were GONE.
So in summary, yes copper bore brushes do put deposits of copper in or on your barrel, but it is only copper, and copper cleaner removes it. Sorry, but I was not able to get any lasting scratches.
Why does the Marine Corps Sniper guide suggest brushing with a copper bore brush at least 10 times per cleaning? So that the accuracy drops and the sniper misses his target? Nope. Is it so they have to replace barrels more often, often working under considerable budget constraints? I seriously doubt it. The Military wants to win, kill the bad guys, use as few bullets as possible, as few barrels (or say M-24 or M-40 rifles), and keep the soldiers alive.
There is an Army Marksmanship Unit, and beyond competitive shooting, they test and manufacture rifles. They do tests on everything. You might say that they just want to keep m-4s clean at all costs and quickly, but they are not going to recommend bronze brushes if they damage expensive sniper rifles. They do tons of tests.
I was re-zeroing my AR the other day, getting it as accurate as possible at 55 yards (the closest range I was shooting at the time - if you start your zero closer, you will use less ammo, and have to do less walking. After I got it dialed in, I wanted to compare ammo. I very rarely do field cleaning when only firing a few mags, but I did want to so I could compare ammo. PMC X-TAC 5.56 XP193 (by the way, why is it XP193 instead of XM193?)
It is 5.56, called X-TAC (and cost the same as the plain boxed PMC but it has a picture of what appears to be an elite unit with three men with weapons and lights) - I got the distinct impression by the name and picture that they were trying to sell this as premium tactical ammo. The Federal is made in the good old USA, while the PMC is made in RO (south) Korea. Better than a communist manufacturer, but what if DPRK is supplying ROK with the components. North Korea hates the US just about as much as China, Russia, or any of our favorite non-NATO countries. The lunatic in charge of DPRK wants to destroy us, so why not start by supplying us with ammo that will help them defeat us. I am going to do some more rigid testing. While I do prefer more powerful ammunition likely to fragment better and cause traumatic injury, I do tend to favor accuracy over raw speed.
I don't know why the PM instead of XM unless it is for practice, but after using the butt-stock cleaning kit in my AR, this shot 4 holes as a ragged cloverleaf and one about a half inch away, I realize that is not spectacular at 55 yards, but it was more accurate than than the Federal XM193, although the Federal had significantly more muzzle flash, bang, and shot about 1.5 inches higher, so it is obviously loaded much hotter. I did shoot 10 rounds of the PMC before switching to the Federal, so that theoretically could have caused the drop in accuracy, but at 10 rounds? The first 5 shot group after the field cleaning was the best group of the day, so I am going to have to re-think some things. Anyone who knows anything about this feel free to write back or PM me (although I sometimes go a while between logging on).
I realize this post is old, but I was searching on how to clean bronze brushes during cleaning, so that it does not re-introduce fouling into my seemingly un-cleanable Glock barrel. I can't get it clean to save my life. Never ever been able to get patches to come out completely clean, especially after brushing, but I have a brand new brush, and have cleaned it three times since the last time I shot just one magazine (13+1). It looks as shiny as a mirror inside. No apparent powder or copper fouling at all. So shiny, but never perfectly clean according to patches. For once, I want to clean it so that no matter what, every patch comes out clean. I didn't mean to get off topic here, sorry.
In conclusion, I will continue to use bronze brushes, but will always follow up with patches with copper solvent, and in doing so, remove the fine "scratches".