There have been times through out my 30yrs of loading and developing loads for the range and hunting, that I found my groups opening up and struggled to get the accuracy I wanted. Through the years I have discovered one of the most over looked culprits leading to poor accuracy.
Carbon is much more of problem in all the load work I have done through the years than is copper. We get to thinking copper and everybody attacks the copper. Yes there will be copper buildup. Some rifles of mine had rough and poorer barrels than others did and with the soft copper Barnes bullets they would build up quicker than the good barrels. I assumed for a while that it was copper build up that was causing my groups at times to open up some. So I changed bullets and the same thing would begin to happen, even though I had less copper in the barrel when I cleaned the barrel using a different brand of bullet, but why the opening up of my groups???
So I decided to address the carbon issue since carbon is much harder than copper and sometimes is not removed by copper cleaners. Low and behold it was the carbon build up that was creating my problems and then you add the copper build up on top of that, we tend to think or assume it is the copper causing the problem.
Every time you lay down a copper layer you lay a carbon layer. You can end up with what some call carbon rings building up in a rifle and if so you could even have increased pressure signs without having max loads, while diminishing accuracy. The main carbon build up will be just a little ways in front of the throat for a short distance down the barrel and this must always be addressed if one wants to have consistent groups. You add a soft copper bullet like Barnes to the mix and you have a compounded problem especially in a rough or poorly cut barrel.
Also, different powders have different make ups so they build up differently. Hotter powders (those that burn hotter not necessarily faster or slower than other powders but actually heat up the barrel quicker) are the ones I have found to create the carbon problem. Now all that said, I would use three different cleaners in the past to clean my barrels, so as to address the carbon and copper layering problem that takes place in a barrel. Also, I do not over clean my barrels. Now this is not an advertisement for Deaton's gun care!! What it is, is the statement that all I have to use to address both problems that do exist no matter what rifle or barrel I am using, is Deaton's and it does the job.
I don't recall in my life time of shooting and hunting and developing loads all the rounds I have loaded, and for all kinds of cartridges, nor the number I have put down range or used in the field. I do know from this experience that carbon is the underlying culprit that is over looked and has caused guys to give up on loads and even give up on rifles because they could not figure out what was causing them to have accuracy problems.
I have had some ask me how I always get a rifle to shoot fairly good or real good no matter what rifle I bought or custom I had made. Well, it is understanding through the years that both carbon and copper must be addressed and not to assume it is one or the other without also addressing the powders used.
Nothing scientific just years of doing it and getting a feel for bullets, powders, primers, brass, barrel differences. Most rifles whether bought from the factory or custom made will out shoot the majority of us if not all of us, and we should try to come to an understanding of what a rifle likes and dislikes. But lets remember carbon is a sneaky culprit that does cause a bunch of problems at times.