Most centerfire ammunition produced prior to the late 1950's used mercuric primers, and that resulted in more rusted and pitted barrels than any other factor.
Lots of military ammunition continued using mercuric primers even longer (1960's certainly, possibly 1970's as well). I was once placed in charge of a work detail at an Army post armory where we did nothing but disassemble, clean, and reassemble weapons for weeks. Rifles, pistols, machineguns, both US and foreign dating from WW2 through the 1960's. These were weapons used for specialized training, so no individual soldier had been responsible for cleaning and rust in the bores was quite common.
Most could be adequately cleaned using a dry bronze bore brush, then a bore brush with solvent. Those that were particularly difficult brought out the worn bronze brushes wrapped in steel wool, scrubbed without solvent. Then the solvent was applied, followed by normal cleaning. All were checked for throat erosion and muzzle erosion prior to being returned to the racks for next use. Very, very few were found to be out of spec, and no signs of damage were found due to this cleaning method.
Since that time I have bought, sold, and collected dozens of antique firearms, both military and civilian. I have used the same cleaning method repeatedly with excellent results.
Steel wool will not remove pitting. But it will remove surface rust, and do so without damaging bluing or nickel plating.
