I think Lee did make a .310 sizing die at one point. I have one, but for all I know somebody ordered it from them special or it was a group buy (when we used to use Lee). I've opened a few up, both with the sandpaper method outlined above or had them opened up with a lathe.
I think that the Lee (and Star, etc) is a superior way to size. However, probably 99% of my sized boolits get mashed thru a traditional sizer, for simplicities sake. For sizing/lubing truly concentric boolits, the method mentioned above is about perfect....just slow for high volume shooting.