I also just noticed in that same book the 243 Catbird.
It is the .270 Win necked down to 6mm and then fireformed into a Ackley Improved configuration.
So basically a 6mm-06 Ackley Improved, or perhaps to be a bit more precise, a 6mm-270 Ackley Improved.
They say it can get 3400 fps on the Berger 115 gr. VLD, a 100 fps improvement over the 6mm-06.
They do warn that this chambering is very hard on barrels. 1500 rounds at best, even when the shooter does not overheat the barrel. :(
It also requires a very long barrel to reach it's potential. They don't say how long, but I would guess 28" plus.