I have read gunplumers suggestion but want to know if anyone has a better idea.
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Put it in a lathe and face it off at a 11 degree angle with a facing tool. [heavy]

I mean to say without a lathe

Brownells sells a recrowning tool that fits the bore.  Some guys report not having very good results with it, though.

If you just have to touch it up, I've heard of using a stove bolt or the like in a drill coated with valve grinding compound.

If you plan to do only one in a great while, have a gunsmith do it.  This thing is critical to accuracy and is one of those jobs left to experts for a keeper barrel. Sorta like stoning triggers.    

If you're asking questions like this, You need a Brownell's catalog. They have every gunsmithing tool or cool gunpart you can think of.
www.brownells.com
I think they have something that uses the bore as a guide.
If you have a friend who's a FFL. cop. gunsmith, or Dept. Armorer, you can get it wholesale.