Howdy
If Black Powder fouling gets infused with oil, it will not cause corrosion.
I discovered this a long time ago.
Think of it as saturating a dry sponge with water. Once the sponge is saturated, it can not absorb any more water.
If the extra dry fouling from BP is not allowed to absorb any atmospheric moisture because it is already saturated with oil, it does not cause corrosion.
The other factor is we use non-corrosive primers these days. Black Powder fouling coupled with corrosive primers could cause rust. BP fouling with modern non-corrosive primers is not as corrosive as it was with corrosive primers.
Granted, I don't shoot Cap & Ball much, but I only shoot cartridges loaded with Black Powder in CAS. But the procedure is much the same.
Before I begin shooting a revolver with Black Powder I completely disassemble it, and remove all the oils right down to bare metal. Then I relube the entire gun and all the parts with Ballistol before reassembling the gun. This leaces a nice coating of Ballistol down inside the gun.
For a cleaning solution, I use a 1/1/1 mix of Murphy's Oil Soap, Rubbing Alcohol, and drugstore Hydrogen Peroxide This stuff is often referred to as Murphy's Mix*. When I clean the gun, I remove the cylinder and run cleaning patches saturated with Murphy's Mix through each chamber and the bore. Then I wipe up the MMix with dry patches, and soak the bore and chambers with Ballistol. Then I mop up the extra Ballistol with a dry patch, leaving behind a light coating of Ballistol. Same thing with the frame, I wipe it down with MMix, followed by a light coating of Ballistol. The last thing I do is take some Q-Tips and soak them in Ballistol, and squirt some down inside the slot where the hand pokes out of the frame, and down inside the frame where the hammer sits. This refreshes the Oil coating down in side the gun. The entire process takes about ten minutes per revolver, and does not risk cross threading any of the screws. Yes, I can take a Colt apart almost blindfolded, but I don't do so unless it is completely necessary.
This photo is of one of my Colts. I had to take it apart for some routine maintenance. You will notice a heavy deposit of black, oily gunk on the trigger guard, trigger, and hand. You will not notice any rust. This Colt had not been taken apart for at least two years, maybe more. Testimony to how oil infused BP fouling will not cause rust.
You may have a problem leaving your guns this gunky inside if your old drill instructor is still whispering in your ear about never putting a way a dirty gun. I don't listen to such whispers and sleep fine every night.
*Inevitably someone will say just plain water works fine to clean BP fouling, why go to the trouble to make up fancy solutions when water will work just fine.
Two reasons.
1. With water you have to get all the water out again. If you don't get all the water out, you are asking for corrosion to happen inside the gun. That's why shooters use hot water or dry the gun off in a low temperature oven. When I was a kid taking my C&B revolvers completely apart after every time I fired them, inevitably the heated metal from the oven or just from hot water would cause flash rust to form. When Murphy's Mix dries, it leaves behind an oily coating of the Murphys Oil Soap on all the metal surfaces. The oil coating will prevent the BP fouling from absorbing any moisture from the air, and there is never any rust down inside when I do get around to cleaning out all the gunk. So I don't bother to try to get all the MMix out, I just leave it in there.
2. MMix can be used cold. It does not have to be heated. So I can clean my guns right at the car, don't have to wait to go home and heat anything up.