Your stated bullet diameters are correct. The .30-06 case is slightly shorter than the .270 Winchester (2.494 vs 2.56), the difference being entirely in the longer neck on the .270. I had read recently that the .270 was developed from the .30-03 Springfield cartridge case (the direct parent of the -06), which was .070" longer than the .30-06. Maybe this is the "creation mystery" that was being referred to? I know it was always a question mark to me, so......I am pretty sure I am correct on the diameter of all of them. You stated " A 6.8/.277 (Actually 7.0mm) bullet". That is not correct. A 6.8 is a .277 diameter bullet, a 7mm is a .284. That was the point I was trying to make. The .270 Winchester is the same diameter as the 6.8 SPC. What is this .270 Winchester "creation mystery" you speak of? It is simply a 30 06 case necked down to .277. No mystery that I know of.