I had a 6.5 x 57 R in a single shot break action German rifle years ago, great cartridge, shot NT boar, wild dogs, Fallow deer and a handful of buffalo (selected shots) quite successfully. Used the 160 grain round nose Hornady, now sadly not available! I also had a supply of 160 gn round nose steel solids from a military cartridge, maybe a dutch round, that worked well on the occasional big buffalo bull that turned up in unexpected places.

Wish I still had it!

cheers

Chris