As far as I can determine by filling up the cases even with the top of the neck and using the weight method, both the 6.5 Creedmore and M-S cases hold close to 51 grain water.

This is nice because there is a lot of new reloading data on the Creedmore using modern components. Just remember that the Creedmore is designed around a working pressure of 62kpsi and the M-S 52kpsi. Most of the Creedmore starting loads are in the 48kpsi range, so a good place to start is a grain or two under starting and to stop about a grain over it. I found out my 6.5 M-S loves Varget, 4350 and RX17 with the 140 grainers.