Carefully. It's also referred to as striking. Because you can't turn a barrel as thin as many old shotgun barrels without twisting them off they were turned and then struck, that this draw filed, to the appropriate contour. The facets are very, very narrow and are then removed with abrasive cloth, if you're doing the job totally by hand, or on a large felt wheel with abrasive.
With some practice this isn't nearly as bad to do as it sounds. When I'm refinishing a set of shotgun barrels that have some nicks in them, I simply draw file the nicks out on a long taper. Can't be seen in the finished job.
Tom
