is the parent round. The 7 x 57 mm Mauser was necked down to .257 to make the .257 "Bob". When necked down to 6mm, it became the 6mm Remington.
Therefore, the family tree is 7x57, or .284 X 57
.257x57,
6x57, or .243x57
Short actions are too short for them, long actions too long, but usually used, anyway.
That rifle is too beautiful for words. I would take out a second lein on my home to raise money before I would sell that rifle.
P.S. Is it a push feed or a controlled round feed?