Well, here's the good news.....RCBS make the dies. Indeed the gunshop that used to be near Witney, or somewhere there, it was in an old car showroom I think, had a set. So they are out there.
The next good news is that, in fact, there was an "issue" with some H & H barrels for the .240 Apex.
The issue is somewhat complicated. Nevertheless what it does mean is that using a bullet for the common or garden .243 in a .240 Apex should, in fact, give reasonable accuracy.
It is also possible that .240 Weatherby Magnum cases can be sized down through .240 Apex sizing dies to emerge as .240 Apex cases.
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Hope it helps.