How about the 6 BR? Longer barrel life, super accurate, minimal recoil, and great brass available. Would fit your SA magazine even with long bullets. You could even have your exising barrel rebored and rifled if you wanted instead of buying a new barrel.
Link for 6BR: 6mmBR Cartridge Guide within AccurateShooter.com
Quote from 6BR link:
"The 6BR is superbly accurate, very efficient, and very versatile. A 6BR can set world records at 600 yards (and even 1000), yet it is easy to tune, and it is just as well suited for a long day in the varmint fields."
link for reboring/rifling: Classic Barrel Works