PSI is a measurement of applied force is it not? If the PSI is the the same then and now explain the difference between then and now in regards to velocity?
Remember it's the peak pressure that we control for (part of the SAAMI spec for each cartridge) from a safety point of view. Peak pressure is what is damaging things in most cases. But the resulting velocity is not directly related to peak pressure. Velocity is proportional to the integral of the pressure vs time curve (impulse) or proportional to the square-root of the pressure vs distance curve (work/energy). So though a fast and slow powder might both produce similar peak pressures the slower burning powder will sustain that high pressure over the duration/distance of the bullet's travel down barrel resulting in higher velocity/momentum/energy.
The next part of my rambling is based on Quickloads an internal ballistics simulation software I have. I have not worked this particular 357 Magnum load in detail so it may not be absolutely correct but it will be close and hopefully a good illustration of how different burn rates of propellant can produce similar peak pressure but result in fairly different velocities due to the burn rate and differences in the pressure vs time/distance curves.
So lets take a 357 Magnum case, load with with a 158 gr JHP loaded to 1.59 inch OAL and fired in a 6-inch barrel.
In this first example we stuff that case with a maximum recommend charge of Winchester 296 (Hodgdon H110). W296 is a relatively slow pistol powder popular for magnum revolver cartridges among other uses.
You can see here a simulated peak pressure (Red curve) of 34,262 psi and an expected muzzle velocity of 1423 fps. Notice here that we have a muzzle exit pressure of 8826 psi.
Next we take that exact same cartridge/gun but instead of loading it with W296 we use Titegroup. Titegroup is a very fast burning pistol powder more popular with small volume cartridges like 9mm and 40S&W but Hodgdon publishes a max load of 6.1 gr under a 158gr bullet in 357 Mag.
You can see here a simulated peak pressure of 33,108 psi (96.6% of the W296 load) and yet we only have a velocity of 1116 fps (78.4% of the W296 load). Also notice that we have a muzzle exit pressure of only 3522 psi.
So both loads have very similar peak pressures and I think if you look at both of those pressure vs distance curves you can see how much more area (the integral) there is under the W296 curve than the Titegroup curve. Look how fast the pressure drops after the peak pressure is achieve. The Titegroup has completely burned the entire powder charge before the bullet has move and inch. The W296 is still burning all the way to the muzzle (actually wasting nearly 27% of the charge) The area, in this case since its pressuve vs distance, is proportional to the work done on the bullet and the resulting kinetics energy the bullet exits with and we see the W296 load produces 710 ft-lbs of energy and the Titegroup only produces 437 ft-lbs (61.5%).
Now you might think well lets use even slower powders but there is a sweet spot (W296 is pretty close to it) and if you go slower you find it burns so slow that you can't generate pressure faster than the volume is expanding down the barrel and you never generate any significant pressure. So for fun I filled the 357 Mag case full (slightly compressed even) of Hodgdon H1000 a slow rifle powder frequently used in big magnum rifles cartridges like 300 RUM and 300 Norma Mag.
I think its obvious here that the propellent is too slow we never get any real pressure and thus no velocity or energy. In fact if you look in the Results Box at the bottom you can see that you only burn 12% of the powder before the bullet exits the muzzle. At pressure this low the powder might actually stop burning and quib a round in the barrel. I would not fully trust these result as this is likely outside good and proper use of the software.
And the point of all this rambling is that today we have a huge variety of powders available and with these greater selections we can select powders that stay under our peak pressure, stipulated by safety, while maintaining as high a pressure as possible for the rest of the trip down the barrel. In the 1930's when 357 Magnum was invented they did not have as many option for powders and they did not have the pressure measurement equipment and software we have now to help up pick these optimal propellants. There was significantly more trail and error need in load development back then, than is required now.
Combining our larger selection of powders, greater ability to measure, and ability to simulate ahead of time means we can stay within the same safety constraints and produce greater output.
-rambin'
