You can't use a lead boolit that's too big! (or it just won't chamber) .358" is the normal size for lead in a .357 or .38spl. Too small and it may lead the barrel and or not shoot as well. A thousandth or more over the actuall barrel groove diameter is standard for using cast lead bullets. A .308 would get a .309 or .310" etc. Slugging your bore/throat/cone lets you know for sure what your gun is then you go from there.