Just bought a double barrel Baikal hammer gun off "wabbitbosher" Kettering, pristine bores (good price), but heavily rusted barrels. I looked all over the internet looking for something to use, all sorts of chemicals available, at all sorts of prices. I came across a short posted article by an American guy saying he used an old US copper penny. As all our copper pennies had been recalled by the Royal mint, when the copper price went through the roof, I decided to use a short length of 22mm copper pipe, hammered over a mandrel and it fitted my barrels profile. Within 40 mins of rubbing with gun oil over a very rusty barrels, they came up relay well, will not need to re-blue them either. Hope this helps you guys
Here's the original YouTube item: