Shotgun barrels tend to be made from thin metal that can be susceptible to denting. Fortunately, there are two effective ways to raise the dent. You can use a hammer and metal plugs or a hydraulic dent raiser. Both methods have advantages and disadvantages that relate to cost and technique difficulty.
The most basic and straightforward method of lifting a shotgun dent is using a metal plug that slides into the shotgun barrel. Use a metal rod to slide the plug into the barrel until it fits directly under the dent. Using a light metal hammer, gently tap out the dent. Tapping onto the dent with the plug underneath actually causes the dent to raise. Metal plugs work well, but it is a slow process because you need to insert different-sized plugs to address dents that vary in depth on the shotgun barrel.
Hydraulic Dent Raiser
According to ShootingTimes.com, the hydraulic dent raiser is imported from England and functions by forcing a small anvil at the tip of the long rod upward, which in turn forces the dent out of the barrel. The method of fixing the barrel is similar to metal plugs, and the raiser slides into the barrel until that anvil is aligned directly under the dent. Be sure to work slowly and cautiously with the hydraulic device. You raise the anvil by slowly turning a handle at the end of the raiser. As you turn, ensure that the anvil is aligned with the dent. Be careful not to turn too quickly because you may overexert the raiser and push the dent outward, creating a metal "bubble" on the exterior of the gun. The tool can be quite expensive, almost the cost of a new rifle barrel. Nonetheless, if you have repeated dents, it is better to have a tool to fix the dent rather than continually purchasing a new barrel.
Finishing the Dent
After you have successfully raised the dent, you need to bore or polish the inside of the gun barrel to remove any traces of the dent. ShootingTimes.com recommends using a Flex-Hone to slide into the barrel and polish the inside. The Flex-Hone is a rod attached to a cylindrical steel tip that looks much like a round metal file. It fits just inside of the steel barrel and if slid back and forth under where the dent used to be it will polish the barrel and ensure there are no remnants of the dent remaining.