The 7 x 57 is a great round and one of the iconic centerfire cartridges that has been with us since the late 1800's.
The OP is correct about the twist rate of his M77 being suitable for heavier bullets (170 -175 grain). While many of the newer rifles chambered in 7mm Mauser will shoot the lighter grain bullets (130-150) the older military rifles and the ones with the faster twist rates, like that M77, will generally shoot the heavier ones better.
The pressures for factory loaded 7 x 57 ammo are held a little low due to the older small ring Mauser rifles that are out there. However, with the 173gr soft point the factory ammo 7 x 57 will perform very well. In fact, that round has a LONG track record of success.
As the OP found out, the R
in the 7 x 57R
stands for R
The 7mm Mauser or 7 x 57 is a rimless bottlenecked cartridge that headspaces on the cartridge shoulder.
I would recommend the OP shoot different bullet weights in his M77 to confirm what produces the best accuracy but my guess is the 173 grain will be the winner.
The 7 x 57 has almost been killed off by rifle manufacturers, which is a shame. While ammo is available for the 7mm Mauser, there aren't many manufacturers making new rifles chambered in that fine cartridge.
Federal makes a fine 7mm Mauser cartridge with a 175 grain round nose soft point: DSCN1731.JPG
Good Luck !