The model 13 is it's own model, designed to fire .357 magnum, like other K-frame .357s. There was actually a Model 10 that was adapted to fire .357 magnum. You can find reference to it in some of the model 13 threads, it's an interesting subject. There's lots of us that are near crazy for K-frame .357s. I'm definately one of them. Think of your model 13 as a blued version of a model 65 and maybe that'll put it into perspective. Oh yeah, and congratulations on finding one. Post pics when you get it, please!