OzFclass

shaun,
the 6.5 x 55 and the 6.5 by 57 are virtually identical in case capacity, because although slightly different lengths, they are slightly different diameters as well.
the 6.5 x 57 necked down to 6mm is the 6mm Remington.
this would give you a ballpark figure.
the 6.5 x 55 would gain a lot of capacity if ackleyized, due to a lot of taper removal.
this could be a bit tough on barrels, and the expense of tooling might be regretted.
the 6.5 x 55 is not to be sneezed at in good hands.
a good 6mm cartridge night also be the 6mm super lr.
keep safe,
bruce.

Bruce,

Cheers mate. I've been discussing this all with my father in law today. We just returned from the range seeing how well his old 6.5x55 hunting rifle shoots. He asked the questions and I couldn't answer but I knew you all here could help me out. Ill pass on the info to him.

Thanks a bunch mate.