While not an expert I can tell you Sellier and Bellot 173 grain loads are fairly hot. Both Norma and RWS are lower velocity so if the S&B is shooting high try the other two. The velocities are published on the websites.
I handload for my 7x57R and use a scope. It is a very accurate cartridge, easy to load for, and at least in my gun, anything from 140s to 175s shoot very good groups. The only issue would be adjusting the velocity of whichever bullet you wanted to use, which is easily done by adjusting the powder charge,to get it to match the open sights.
For break open guns you may want to look at Hybrid 100-V. It has very much lower pressures at the same or better velocity than most of the powders commonly used in the 7x57s.
