Gazz
Pin gauges are perfect for the job.

I've used my friend's cheap set and simply rounded one end and polished that radius, grease it, and drive it under the dent, then tap away. I would advise against using a significantly undersize plug as you may end up printing an edge or tigher radius on the opposite side. I've alway used a matched size plug to the bore and drove it under with grease on it in one shot. The barrels are much softer than most people think. Go for it.