I would personally go with a 6.5x284 - or a 6.5x57IMP.
SA 60 gr. case chamber can easily be cut with a .260 Rem chambering reamer, just run it in far enough to headspace on a necked down 7x57 or 8x57 case.

The resulting ctg. will have a .454" shoulder, although the short neck of the .260 Rem- not a bad thing at all.

My daughter's rifle chambered for this (initially) made 3,300fps with 120gr. Barnes XLC's using Reloader #19.