X-Mirror Weights

I’ve been weight-painting a model, and although I’ve had x-mirror on, it has generally not been happening. When a single bone sits on the centerline of the body, it works most of the time, but not perfectly because the center of the torso is not symmetrical (he’s wearing a vest). The hands, on the other hand, are symmetrical. I thought that the problem might be a naming convention thing.

I know that bone position mirroring works if you name them either as blah_L and blah_R, or if you do blah.L and blah.R. I was using the blah_L style, and some things that I’ve read since seem to suggest that weight mirroring only works with blah.L. However, after changing all the names, any additions to weight painting I make are still not mirroring.

But even the new changes aren’t really the issue. In fact, none of it’s really an issue, since I’m nearly finished anyway. What I want to know, though, is this: is there a way to apply the mirror of already-finished weight painting for blah.L to blah.R? In other words, can I make what I’ve already done to the left side of the body happen to the right side? Can I make those right-side bones pick up what I’ve already painted around their sister bones on the left?