Jack, I made an expanding dent raiser of the type you mention, about 10 years ago, and have used it several times. The idea came from a sketch in Howe's "Modern Gunsmithing."

It works very well, and most of the tips given in this thread apply to it (taking it easy and not going too far, hammer taps around the edge of the dent, lots of grease). Lacking a brass hammer I used a steel one on a piece of brass shim stock when I did a barrel I didn't want to refinish.

Fred