In that same thread:
Several reasons the 7mm RM is "underloaded," aside from the primary reason posted by Mathman, widely varying pressures:
The original factory loads were chronographed in 26" barrels, common back in 1962 when it was introduced, with a powder then unavailable to handloaders, IMR7828--which even today performs alongside newer magic powders.
This allowed the advertised factory velocities, but even then Remington factory ammo didn't quite make them, according to the Speer #6 manual, which contains a section on chronographed factory loads.
Also, the early reputation of the 7mm RM may not have been established because of actual ballistics. A lot of hunters and handloaders back then didn't have chronographs, so accepted the published factory and handloading velocities, yet their rifles still killed game with "reduced" ballistics.
A good friend bought one way back when, when everybody here in Montana had to have a "Big Seven," and according to the loading manuals his 160-grain handloads got over 3000 fps. It knocked the snot out of everything from deer to black bear to elk.
He bought a chronograph a few years later, long before most people did, and found out his magic handload got around 2700 fps, pretty much warmly-loaded 7x57 velocities.
But then, the 7x57 has been knocking the snot out of big game for a long time now!
MD is referring to this:
Pressure isn't constant shot to shot. Modern testing equipment is able to show that the probability distribution of the pressure for the 7mm Rem Mag is spread out somewhat more than other cartridges. So the loading was changed to lower the mean pressure of the distribution enough so the area in the no go portion of the high pressure tail of the distribution became acceptably small.
That's what I consider "erratic pressures" with respect to a particular cartridge, in this case the 7mm RM.
Dimensional differences gun to gun causing them to need different powder charges will affect any cartridge.
