My 12" Ruger Old Armies shoot well with a 1/4" grease cookie made of Gatofeo's lube under the ball, or a felt wad saturated with Gatofeo's lube under the ball. Even so, I run a wet patch down the barrel when we break for lunch, since I have 45 minutes or so until the next stage.
I lube the cylinder pin, the recoil shield, and the sides of the hammer with bore butter prior to the shoot. Makes clean-up easier.
Since I pull the cylinder at the unloading table to load up using a Powder Inc. cylinder loader, I usually wipe the cylinder face and back of the barrel after each stage. I can also take care of any fouling on the cylinder pin at the same time. Spray a little Ballistol on the pin & she's good to go.
Keeping a 12" barrel free of fouling is all about the lube.
You need a good lube & lots of it.
Gatofeo's recipe is the best I've used.
2 parts canning paraffin (grocery/hardware store)
2 parts mutton tallow (Dixie Gun Works or make your own from lamb fat)
1 part beeswax (local beekeeper)
All by weight. Melt together in a double boiler.
Here's my 12" ROAs in action -- A real hoot to shoot!
--Dawg