The right thing to do is to slug your barrel (search the forum, there will be threads on it), and then size your bullets .001 over what your barrel measures.
And now I must confess that I have never done this with my 686. Mainly because the guy who introduced me to casting, had always sized to .357 and never had any problems with leading or poor accuracy. We use a rather hard alloy, that could be the reason for this.