I have a couple sets of pin or plug gages that were made with a slight bevel, so they can be used for barrel dent removal work without causing further damage to the bore. They are also .501" to .625" and .626" to .750". However, as David notes, many sets are not beveled, and would need to be altered for use as dent removal plugs.
Although I've never tried using heat to aid in barrel dent removal, I can believe it would work. I clearly recall learning about how steel properties can change significantly with minor heating when I first started working at a steel mill. I was working on a Slitter, which was used to slit roughly 60" wide coils of flat rolled steel weighing upwards of 20 tons into narrow strips of various widths. One order we had for a few thousand tons was some high carbon high alloy steel that was very hard and springy. It was during a severe cold spell when outdoor temperatures were around zero degrees F, and it wasn't much warmer inside the mill. This particular steel alloy was giving us fits because it was chipping on the slit edges, creating quality issues, and the scrap kept breaking like an icicle. In fact, I still have a small hairless scar on my right shin from the scrap strip breaking and springing back, and slicing into my leg.
Apparently, they had this problem in the past, because it was decided to fire up the gas burners under a large insulated tank of water, and heat it to near boiling. When it was hot, the overhead crane would set ice cold coils into the hot water and let them simmer for hours. After the steel was heated by perhaps 150 degrees or so, it behaved like a different animal, and ran through the Slitter much better and easier. Of course, this extra heating step was troublesome and consumed a lot more time, so we were happy to finally finish that order. But it taught me that steel properties can change quite a bit at temperatures far lower than we typically associate with annealing, heat treating, or tempering.
On the other hand, there was another alloy we made that was called helmet stock or grenade steel. It had been used to make Army helmets before they changed to Kevlar. It was also used to make cluster bombs, and a pattern was rolled into it by Defense contractors to help it fragment into small pieces of shrapnel when the bombs or grenades detonated. That stuff was extremely brittle and would sometimes shatter even when it was still glowing red hot as it was being coiled up coming off of the Hot Mill. The many holes in the sheet metal walls at the Downcoiler end of the building told you that it wasn't a safe place to be when we were running grenade steel.