Dutch: When I served as the "Rammer" and cleaner of my friend's 6-lb. Field piece, I had a cleaning rod that had both a wet and dry end to it, used one after the other, to put out burning embers, and then pull out any excess water in the cannon barrel. I kept dry rags to wipe the moisture off the drying swab( made of sheepskins with the fur left on) and I routinely wiped the wet wool fur end off after it came out of the bucket to limit the amount of water that went down the barrel.
I suspect that the rusting was as you describe, but since the gun was " government property" the cannon crew was not particularly concerned with upkeep and maintenance. I doubt those barrels ever saw the hint of grease to protect the barrel from rust between battles, and no one gave a tinker's darn about rain, sleet, snow, or hail getting into the barrel.
The 6-lb Field Piece my friend built had a 1/4" stainless steel liner in the cast steel barrel. He oiled the bore nonetheless, after it was cleaned for transportation and storage. And, we covered the gun and muzzle with tarps in transit.
For safety, we wrapped the powder charges in alum. foil, and used a brass spike to run down the touchhole to pierce the foil packet to fire off the charge with either fuse, or priming powder. Use of priming powder was far more exciting. :wink: :hatsoff: