I am fortunate to have a small metal lathe to make plugs for removing dents and other things. What I do is measure the internal diameter of the barrel right before the dent, (since there are no gages to measure a.410 you could use a plug diameter of .410 and see if that fits and go from there. There are a few companies that make hardened plug gages but they do no have a beveled edge and I would not try them in the bores of any gun. I then make a tapered plug so that the front taper passes under the dent and the main body of the plug is .001-002 smaller that the diameter of the barrel. On the back end of the plug I drill and tap for 1/4x20 so that I can thread a rod into it. I then heat the barrel with a propane torch till warm and then tap the plug till it goes through the dent area.
You can also leave plug where it gets tight in the barrel and tap the outside with a small fender hammer, the one with the large flathead. Like stated by Keith I have never done a .410 but shouldn't be any different than any other guage.
