Google electronic bore cleaners.
Electrolysis rust removers are easy to make at home. A plastic container, water, some scrap steel and Boraxo washing powder. Put rusty part in solution attach positive lead from a car battery charger to the item and the negative to the scrap steel. The item and the scrap must never touch. Line of sight from the rusty item to the scrap steel is best. To rig up a bore rust remover you will need a steel rod to go down the but but care must be taken that it never actually touches the barrel. Might be a tricky set-up but can work.