Yea it is a 257 Roberts case necked down then fire formed in the chamber. When formed it has the same powder capacity as a 30-06 case. The extra powder and long barrel really pay off in velocity. It has a pretty flat trajectory as well.
I have thought about rechambering my 6mm to a sharp shoulder case
but I don't shoot enough anymore to justify it. I have to drive way out to kern county out hwy 33 to shoot without any jackasses around too far to go to shoot.