I own a 1991 vintage Win. M70 in 7mm Rem. Mag. To reach book velocity with 175 grain bullets I have to load something like 4.5 grains over book maximum. Have been shooting the load in question over the years and case life is good. Can't explain the anomaly but my feeling was that I didn't buy a 7mm Rem Mag in order to get 7x57 velocities. It is kind of a moot point now since I haven't hunted with the rifle in years.
Jim
Post 64 M70's in 7 Rem Mag have/had longish throats and blocked magazines restricting OAL to the "gold standard" 30/06 level. Bullets are seated deeper than necessary and jump to the lands.Often, this affects velocities.
I've seen more than one not even start to percolate properly until charges were at or over max.
Same/same with 300 Win Mags. Seen it a bunch of times.
Among the many reasons results vary so much rifle to rifle and guys scratch their heads.
Why is it like that? I think it's so the rifle will function with a wide range of factory ammo under many temps and conditions. Cuts pressure. We won't notice 100 fps difference on an animal at 300 yards but will notice if the bolt freezes up from a too-hot load.
