Hello,
As has been said, the 7x57 is one of the cartridges very easy to increase the factory USA velocities by handloading, staying inside normal pressures. By "normal" pressures I mean modern cartridges pressures, not "blue sky" ones!!! Even to use in the extremely good pre-98 Mausers, made in Germany and Belgium!As an example, NORMA and RWS has ALWAYS loaded the 7x57 to modern .30-06 standard pressures. And this implies there would be some margin to work...
I am doing these since 1975, first in my DWM M93 Original Sporting rifle, with its perfect dimension magazine/rails for the 7x57 (I can chamber 5 empty CASES, not loaded cartridges, from the magazine working the bolt...).
In my experience, all depends on the combination of magazine lenght vs. camber lead lenght.
By design, the original 7 mm Mauser chambers have a rather long lead and a low angle start of the lands in the barrel. Designed for the original 11,25 grams (about 173 grs) round nose military bullets.
Even all the .275 Rigby rifles, nothing more (or less, by the way..) than German Mauser factory barreled, somewhat refined, and stocked by Rigby, had that original long lead chamber for both the 173 grs and the 140 grs HV models. Being the sights regulations the only real differences.
So, going to modern and entirely safe-normal pressure-loads, IN RIFLES WITH THOSE ORIGINAL BARRELS, or C.I.P. standard new barrels, and using standard lenght M98 magazines, the 7x57 cartridges, depending of the bullet used, can be loaded at 83 mm OAL. For example with 150/160/175 grs Nosler Partition bullets.
Using slow powders like 4831's, R19 and/or R22 (my favorite with 160/175 grs bullets), velocities of 2700 with 160 grs and 2600 with the 175 grs, are very easy to reach, and entirely safe, normal pressure loads, with long case life!!
Good luck!
