Tool called a dent raiser, check local gunsmiths first, kind of expensive to buy for one use. Works hydraulically. Insert into barrel to position a little "anvil" at dent location. Set lock ring to maintain position, allen screw in rear of handle is turned until anvil contacts inside of bore. Turning the allen screw slowly and carefully the dent can be raised and will almost disappear on the outside of the barrel, then gently rotate the entire tool inside the bore to burnish the area where the dent "dimple" was. Works great, if you can find someone who has the tool, I wouldn't hesitate. Shouldn't cost that much either. Quick simple operation if you have had experience, good instructions come with the tool if you don't. Each tool only works with one guage, be sure 12 guage is used, as you stated that your gun is a 12.
