if you plan on a diy solution be prepared for alot of work and dont be suprised if you do even more damage. one of my first projects when i became a apprentice machinist was making expanding mandrels for taking dents out of arrows and a set of solid mandrels to fix the barrel of a old winchester single shot. the acutal making of a solid mandrels is fairly easy even on a hobby lathe if you have a source for high speed drill rod of the right diameter. just make sure to slug at least part of your barrel so you know how big the mandrels have to be. the biggest mistake i made was heating up the barrel and using a grease that wasnt meant for high heat. i got the mandrel part way down the dent and the grease basically turned to a grimy sludge, then a baked on film. so if you plan on doing this yourself do it cold and make a sliding or expanding anvil type tool so you dont get stuck half way through. ive also heard that this should be done dry with no lube.