Gentlemen, and Ladys that may be present,
For copper fowling removal, I've been using gelled household ammonia.
Almost a year ago the wife began using a new item to thicken sauces, gravies and the like. She happened to make the statement that the stuff worked so well that she thought a very small amount would thicken up a gallon of water! So, I said (in what turned out to be a lucid moment) how about ammonia?
Well, we attempted to mix about 1/2 - 3/4 teaspoon Xanthan Gum (prounced: zan-thann gum) a powder much like cornstarch, with about 4 ounces of household ammonia. This produced a very nicely gelled substance that adheres well to brushes and the walls of the barrel being cleaned of copper.
Xanthan Gum can be found at any good "yuppie" type food store such as Wellspring/Whole Foods although it is becoming more common in the mainstream food stores, if they have a well-stocked "health foods" section.
To mix pour about 4 ounces household ammonia into a glass bowl and begin vigorously mixing with a whisk type tool. SLOWLY start adding the Xanthan Gum until about 1/2 to 3/4 teaspoon of the powder has been mixed into the ammonia. If you add the Xanthan gum all at once and then attempt to mix it into the ammonia it will clump-up and not thicken the ammonia causing you to go into "do-over" mode.
The ammonia stays gelled at room temperature, and my mixture is still gelled almost a year later!
To use:
Just apply with a brush to the barrel (best not to use a brass brush unless its a nearly dead one as the ammonia will destroy it) let the barrel sit for about 10 to 15 minutes and then swab out the desolved copper and ammonia residue with several water dampened patches. Repeat a second time if the barrel is really badly copper fowled. Finish up with your normal final oiling/rust preventive procedures.