I believe the way the pressures are measured has changed over the years. This dumbing down of loads or the reducing of loads I’ve seen mentioned here. Could someone explain the reason for the differences between the old data and the new?
I believe it has been dumbed down. I have very old reloading catalogs, Phil Sharpe's Reloading book, for example. Some of the powders that he lists are still around. Bullseye, Unique, 2400, maybe IMR 3031 and 4350.
Hercules was very good about supplying pressure data with their reloading data, and in Phil Sharpe's book you can look at the pressures and the powders charge, and the old loads show greater amounts of powder.
Now the old data was taken with copper crushing equipment, and that is not the same as psia.
Most of the old manuals, Lyman, Speer, Sierra, they used their barrels and actions and estimated pressures the good old fashioned way: Blown primers!
Data produced by that measuring technique does of course vary all over the place. I believe, especially in pistols and revolvers, they purchased factory ammo, shot that across the screens, and used the velocities as the upper limits for their load development. I really doubt the old pistol data had any relationship with pressure at all.
Also does the pressure limits on cartridges have to do with the strength of the guns or the strength of the brass?
For new cartridges and guns : Yes, and yes. When cartridges and firearms are designed in tandem, the thickness of the cartridge, the operating pressures are determined, the gun envelope is determined, if both prove satisfactory, then they are baselined.
For rifles, the cartridge is always the weak link. Revolvers, usually. In semiautos, it all depends. In all cases, the gun designer designs the mechanism to support the cartridge. Strong designs support the cartridge better than weak designs. If the cartridge ruptures, than you have a big problem.