The most extreme rusted barrel I took on was a custom percussion gun made in San Francisco in 1858. A lot of collector value, it had a Whitworth style hexagonal bore. The owner wanted it cleaned up enough to show the rifling style and have some eye appeal. I fought it for several days, finally went on YouTube and studied the videos of electrolysis cleaning with a battery charger, washing soda solution. It did a remarkable job without displacing any metal. I used a 1/4" steel rod with nylon washers/insulators as the + anode and the barrel itself as the - cathode. I mixed the water and washing soda ....about twice as strong as recommended at 1 tsp per quart. 6 hours it was done. Changed solution every couple hours. In spite of my advice, the owner shot the thing, it's a wonder the breech plug didn't blow out the back.
