I use Sweets 7.62 brand ammonia based cleaner from time to time and as I recall the directions on the bottle says not to leave the solution in the barrel for more than 15 minuets as it may etch the finish on the barrel. Now this stuff works real good but I use it very seldom and leave it in for only a few minuets at a time before getting it all out. I've used Sweets primairly on my M1A when it used to get near 1000 rounds or so pumped down the pipe in a single session of shooting. (Me and a few other fellows having too much fun sharing it between ourselves in one day)
That being said, I'd like to add that I subscribe to the theory (if it is a theory) that most all the copper that gets in the barrel is coming from the throat of the barrel. You see, by the nature of the process of making and finishing a barrel there are "tooling" marks left in the throat of the barrel. They look like super fine file marks of the surface of the throat of the barrel.
That nano-second or so when the bullet takes up the clearance in the throat before it hits the lands of the barrel, the heat is so intense that some of the copper off the bullet actually turns to a plasma with all the other expanding gasses following the actual bullet as it travels down the barrel. The copper then is falling out of suspension with the other gasses following the bullet down the barrel and sticking to the barrel.
Copper sticks to itself better that it sticks to the barrel and "breaking-in" a barrel with the one shot and clean for several cycles then 3 shots and clean, and so on is really letting the bullets "polish out" the throat and keeping the build up of coppepr in the barrel to a minimum during this process. It's much easier to clean a little copper out at a time than to let it build up and then clean it out. The copper build up never actually goes away, but in time will reduce in amount of indications of it being in the barrel when cleaning.
A hand lapped barrel will usually show reduction of copper build up fairly quick when compaired to a factory barrel which is usually not hand lapped.
Well theres my 2 cents worth and it is my story and I'm sticking to it.
Also, if you measure your COAL of your rifle you will usually find quite a bit of clearance between the throad of the barrel in relation to the bullet. SAMMI specs and all. In my Remmy 700 I had about .013" clearance in this area. I reload for accuracy, so I seat my bullets for this Remmy 700 rifle of mine to about .004" clearance to the throat. Along with improving accuracy (reduced bullet jump), I notice less a problem of natural copper build up in the barrel.
Reloading like this makes the ammuition specific to just that rifle and may be dangerous in another rifle even if of same brand and caliber.