I mostly shoot my two percussion cap Hawken rifles, but the cleaning method should be the same since I also remove the barrels and wash them.
I've been shooting black powder for a little over 3 years now and I've never heard that one shouldn't use HOT water to clean the barrel. In fact, several of the old timers in our Long Rifle Club told me to use the hottest water I can find... and so I do... and I don't have any problem at all with rust.
I remove the barrel from the stock, fill a small pail with EXTREMELY hot water and a small amount of dish washing soap, leave the flash hole open, put the rear of the barrel down in the pail of HOT water, put a patch on my ramrod and work it up and down in the rifle's bore.... causing the hot water to be pulled up the barrel to the muzzle with each stroke of the ramrod. I run the ramrod up and down in the barrel maybe 10 or 15 times, flushing that nasty black debris out of the barrel.
I sit the barrel aside, empty the black water into the toilet, rinse the pail out with super hot water and put enough very hot water into the pail to more than cover the flash-hole, put a clean patch on my jag, put the end of the barrel down into the water and repeat the action of running the ramrod up and down in the barrel to wash out the soap. AFter 10 or 15 "strokes", I lay the barrel aside and dump the water down the toilet and refill the pail to about half way up once again.
Again, I add a new patch to the jag and run the ramrod up and down in the barrel to insure all the soap is cleaned out. By then, the patch is coming out perfectly clean. After another 10 or so "stokes" of the ramrod, I lay the ramrod aside and
use my small air-compressor's 100 pounds of compressed air to blow the barrel, nipple and open screw hole in the rifle's drum clear of all water and moisture. Once the very HOT barrel has been blown dry... and it dries out FAST due to the heat retained in the barrel's steel, I slide my small barrel light down the barrel and inspect the barrel for cleanliness and debris. It has always been totally free of debris and is always shiny, bright clean.
After insuring the nipple and drum are clean and dry by blowing high air pressure into their openings, I use a pressurized can of Ballistol to spray into the flash channel through the empty screw hole in the drum... and I also squirt pressurized Ballistol into the nipple to lubricate and protect it as well.
Then I wet a clean white patch with a generous amount of Ballistol from the larger can of Ballistol. fit the patch into the slotted ramrod tip, screw the slotted tip into my ramrod and slowly run the slotted tip down the barrel, spin it a little and slowly bring the Ballistol patch back up the barrel to the muzzle, spin the rod a little again, and push the ramrod back down to the bottom of the barrel. I repeat this operation until I'm sure I've circled the bore at least TWICE. Then I lay the barrel aside and clean up the wood.
Once everything is clean and oiled, I put the barrel back into the stock and stand the rifle, muzzle DOWN, in the corner with the rifle's muzzle on a folded over paper towel to catch any Ballistol that might drip out of the barrel over the next several days.
I've been cleaning my cap-locks this way for more than 3 years so far and have never had a "rust problem" due to the heat of the water which heats up the steel in the barrel which causes the barrel to dry itself fairly quickly plus the drying effect of the high pressure air from the small, blue Campbell-Hausfeld air-compressor with twin round air tanks that I purchased at Harbor Freight for less than $80 on sale for exactly this purpose... and it works beautifully!
Just prior to leaving the house to go to the rifle range, I use the air-compressor's compressed air to blow any remaining Ballistol out of the flash-channel and nipple and the lower area in the barrel by an extended "blowing" through the nipple and the screw-hole in the drum. Once that is finished, I replace the screw in the threaded clean-out opening in the drum and I'm ready to go shooting.
At the range, I usually shoot one or two percussion caps through the nipple to insure the flash-channel is open and the bottom of the bore is dry. Then I load the rifle and begin shooting.
It sounds to me as if someone has told you to use
COLD water... and I don't understand why when several long-time muzzle-loader shooters told me exactly the OPPOSITE... and
HOT water works a LOT better... and the HOTTER, the BETTER!~!~! I believe the COLD water is a big part of your rust-problem. If the steel were HOT rather than COLD, the HOT steel would eliminate the moisture almost immediately once the water was poured out of the barrel. Of course, a good "blowing out" with 100 lbs of compressed air also helps gets rid of the moisture, too.
Jus' my 2¢... I hope it helps you...
Strength & Honor...
Ron T.