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Sweets is very strong ammonia. If it will remove copper from your bore, it will remove copper from the brush. I have used Sweets for years on my BR barrels and never experienced a problem with etching. I use it after cleaning with Kroil/Shooters Choice, and never leave it in the bore longer than 15 min. I apply the Sweetsa with bore mops, which are then washed out for future use.
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Aqueous ammonia strength is measured in density (Baume), with 26 degrees Baume the max under STP. Sweets is pretty weak compared to 26 Baume, but considered strong for most bore/ammoniacal cleaners. Ammonia evaporates pretty quickly, but you shouldn't be using a brass/copper brush with Ammonia.
I agree with FiftyDriver. Use Sweets or other ammoniacal product if you have a clearly identified copper problem.
I've talked to several metallurgical engineers who agree with FiftyDriver and insist ammonia will etch stainless steel. I've also consulted the most famous ammoniacal chemist on the planet(with my experiment to prove/disprove this assertion). The first thing he pointed out was all the flaws in my experiment and each new proposed one. We finally came up with a good test. When pressed for his opinion "will ammonia etch stainless steel", his only response "do the test". Since that time, I stick with Butch's Bore Shine
Altho BBS has ammonia, it's not nearly as caustic as sweets. Here is what my favorite guru (who once sent me an email once saying Kirby was a great gunsmith & would do an excellent job with my 3 new barrels)
Our preferred cleaning solvent is Butch's Bore Shine made by BBS Industries (406-652-2495). This solvent is excellent at attacking both powder and copper fouling. We recommend it for both break-in and regular cleaning.
For occasional use only, abrasives like J-B paste, Flitz, or RemClean can be used.
Do not use a stainless steel brush in your barrel under any circumstances.
Do not apply a strong copper remover, like Sweets, on a bronze brush. It will ruin the brush and give the false indication that the barrel has copper in it.
Lilja on Barrel Maintenance
For me it doesn't get any better than Lilja. Engineer, BR shooter, LRH - everything.