As long as you stay away from the +P loadings for the .257 Roberts you should be just fine.  I looked up the pressures at this link   (You need to Login; The only other concerns I would have is overall cartridge length and rim thickness for the extractor (the 6.5x55 Mauser round had a much thicker "rim" than the 7.92x57 it was developed from). The thinner rim of the .257 Roberts could have problems with the extractor being able to hold it properly.