I can't see anywhere on the internet that will show the shoulder datum of the 260. But we can find the 308, 7mm-08, and the 243.
But we can infer from this it is the same old 1.630" min 1.640" max from the chamber shoulder where it is 0.4000" in diameter to the breech face. http://www.clymertool.com/headspac/index.html