That's the problem I had with my DCM Garand. Over the National Match Course, a 50 round old standard competition, the last 20 rounds are 600 yards slow fire, so the part that demands the most of the rifle's precision is at the end, after you've accumulated some fouling. That rifle shot like a house on fire right up until nearly round 40, then the scores would start to drop, going from mostly 10's and X's down to picking up a couple or three 9's, and then in the last 10, 8's and even the odd 7 would show up. It would then take from after dinner all the way to bedtime working with Sweet's 7.62 (a strong ammonia-based cleaner) to get the blue patches to stop coming out for the next day's matches.
There were warnings back then not to use ammonia excessively in bores. That didn't hit home for me until I left a die with a stuck case the head broken off (thanks, stuck case remover) soaking in ammonia to etch the brass out. A few weeks later the case was gone and the ammonia solution was a beautiful cobalt blue but the die had a layer of rust brown all over it and was in need of repolishing. That convinced me it wasn't good to do.
Today there is no need to use ammonia. It's not even close to being as fast or effective as the chelating copper removers are anyway. I mostly use BoreTech Eliminator, but for a really tough bore I clean all the carbon out with BoreTech C4 and put in their Cu++ product or KG-12. Both attack copper hard and fast and keep on working a long time. Just give them 20 minutes and most copper will be gone. KG-12 actually keeps working longer and harder, but it doesn't turn green or blue, so you need a borescope or Eliminator or Cu++ to determine when the copper is gone from using it.
I've restored a couple of Mausers to shooting condition that had little visible rifling left by starting with Slip 2000 Carbon Killer. It's the fastest carbon remover, but it isn't odor-free like the Bore Tech stuff. Letting it sit 15 minutes loosens most of the carbon caked up between the lands for brushing out. A couple or three applications and the rifling magically appears. The copper remover goes in and voila!