An old Winchester reloading pamphlet dated 3/80 lists 5.6 grains of THEIR 231 with THEIR 230 grain FMJs, giving 17,000 CUP and 765 fps out of a 5" barrel. Their COL "maximum" was 1.275", but it is not clear if the COL used to actually shoot these data was that long. Of course, they also used Winchester primers, but it is not clear whether the formulation of Winchester primers has remained unchanged for the past 32 years.
So, there is data with the actual bullet and powder combination that you are using which indicates that you are not maxed out with your charge.
You say that your handloads are shooting fine, which I assume means that they feed and eject reliably. So unless the ejected brass seem to go much farther than brass ejected from Winchester's factory loads, I would say that all indications are that your handloads are right where they are supposed to be.
Maybe the guys at the range were thinking about loads for 230 grain lead target bullets?