[Ron Wood]

Once upon a time long ago (no, I'm not starting a fairy tale) I straightened a bent trigger guard on a Luger by clamping a piece of iron pipe in a vise to use as a mandrel/anvil and a rawhide mallet to make it round again. If you have ever done any auto body work you know the process...down it where it is up and up it where it is down. That is pretty much the same technique that David has suggested with the hardwood approach.

The rawhide mallet doesn't create any dents like a steel hammer would. The iron pipe is less "springy" than a wood dowel so it provides a firmer working base. If you are concerned about the pipe marring the trigger guard you can wrap it with electrical tape, but only one layer so it won't act as a cushion.