Bronze brushes every time, measure them with a caliper, often, They are good for perhaps 100 strokes. Often you will find bronze brushes over size.
Brush with Montana Extreme bore solvent, safe to let soak over night if need be.
More aggressive treatments as needed, Montana Extreme copper cream on patches, next Flitz Bore Cleaner in the 7.6 oz bottle. Use the Copper cream and Flitz bore cleaner on Montana Extreme or Issio plastic brushes first, if the carbon subsides, then go to bronze bristle brushes.
Caliber, # of grains fired, # of rounds in between shot strings, # of rounds in between cleanings, barrel quality, all come into play as each barrel is an individual.
A guy shooting a short range benchrest rifle may clean every 7-12 rounds while a guy shooting p. dogs may clean a rifle every 125-300 rounds. Compare those two guys with a guy shooting steel with a 7 mag or even worse a guy using a 338 Lapua shooting steel at very long range. Then you have a guy shooting a 50 bmg shooting 200g of powder. Different applications require different cleaning techniques.