A Sturmgewehr in Kalifornistan? Gavin would have kittens, so I HOPE SO.
My thoughts are that someone used 8 x 57 brass to reform to some other caliber. 308 and 30-06 are the usual victims in these atrocities, but maybe the reformer had lots of 8 x 57 brass laying around. Case length, neck diameter, and shoulder form & dimensions might yield some clues. Anything is possible, given form dies and a case trimmer. Rim diameter rules out a 6.8 x 43, which is in the .425" ballpark (30/32 Rem and 40 S&W/10mm).