...The true 'improved' 7X57 is the 284 Winchester. Improved in the sense that it does indeed add a step of velocity while at the same time fitting a short action receiver, for a lighter handier hunting rig..AS has been said though, the old Mauser round is mighty fine as is. Just my personal preferences for the 284 showing through here. I have rifles of both chamberings and have affection for both.
