I stand corrected. I must've been thinking about 22LR vs. 22 WMR.
I admit that I'm curious as to why. 357 Mag has a lot more powder, and slower-burning, but if it burns outside the barrel, how is it so much faster?!
It has to do with the maximum average pressure, the progressivity of the powder and the area under the pressure/time curve.
Cartridge specifications determine the maximum average pressure. Roughly, this is an average of the peak pressure that occurs within the firearm when cartridges are fired. This pressure peak occurs in the chamber shortly after ignition while the bullet has hardly moved and the combustion chamber is still quite small. As the bullet begins to travel forward, the volume of the combustion chamber increases which has the effect of decreasing pressure -- the gases have more space to expand into. The powder is continuing to burn and produce more expanding gas and that has an effect of increasing pressure. The net result is a rapid increase in pressure and then a more gradual tapering off of the pressure.
357 allows for a greater maximum average pressure than 38. Similarly, 44 Magnum allows for a greater MAP than 44 Special. The greater pressure behnd the bullet results in greater acceleration. 9x19mm allows for the same MAP as 357 Magnum. So how can 357 accelerate the same mass and caliber bullets faster than 9mm?
With 357 we can load the larger case with "slower burning" powders. We often refer to "burn rate" as fast or slow, but the digressivity or progressivity of the burn rate is also important. A digressive powder will reach maximum gas production early and then taper off. A progressive powder will increase its gas production over time. There are two principal ways to achieve these characteristics.
Powders can be made to produce gas at a slower rate by coating them with a deterrent -- a substance that slows the combustion. Gun cotton and nitroglycerin are what produce the energy in the form of rapidly-expanding gasses, but the deterrents slow this process down. Since the powder is producing less gas over time, we can use more of the powder so that we still reach the MAP in the cartridge specification, but after reaching peak pressure, there will be much more nitrocellulose/nitroglycerin-bearing powder still left to burn. This larger volume of powder will continue producing a higher volume of gas over time as the bullet travels down the barrel. The result will be that we reach the same peak pressure, but hold the pressure higher for longer. This means that there is greater pressure behind the bullet for a longer time and the acceleration of the bullet is also greater. All that combustion deterrent and extra powder takes up space though, and we need the extra space in Magnum cases to hold both the deterrent and the extra volume of powder it allows us to load.
Powders combust at the surface of the grains. A grain with a lot of surface area like a sphere or disc-shaped flake will produce a lot of gas at first, but less gas as the size and surface area of the sphere or disc diminishes. These are digressive rate powders. Grains that are cylinderical or disc-shape and have a perforation in the center will have an increasing amount of surface area as they burn. As the perforation in the center enlarges, we go from a surface area only slightly larger than that of the cylinder to one where we have both the outside surface area of the cylinder, and an inside surface area nearly as large as that of the outside. These are progressive rate powders.
Trail Boss is a relatively fast-burning progressive rate powder. Notice the holes in the center of each disc/flake. As the flakes burn from the inside out, the surface area increases. Trail Boss is a bulky powder though, and it will almost always fill as much case space as you have while still producing low MAP.
Extruded (stick) rifle powders are often both progressive (there is at least one perforation in the cylinder-shaped log of powder) and heavily deterred so that they take up a lot of case volume. Even loading a large pistol case like 44 Magnum with these rifle powders won't produce very high peak pressures. They are just the ticket for big bottleneck rifle cases that are loaded into chambers behind long barrels that give plenty of time for gas production behind a bullet before it exits the muzzle.
There is no doubt that short handgun barrels waste a lot of slow-progressive-burning powder, but these powders are still key to producing the greatest velocities for a given maximum average pressure because although they may take a little bit longer time (a couple of millimeters of bullet movement) to reach peak pressure, after they reach that peak, the pressure will drop off more slowly than with a fast-burning powder. Keeping greater pressure behind the bullet for a longer time is what produces a greater acceleration.
There is a noteable side-effect of using these slower burning and more progressive powders. Because the result is a higher level of pressure further into the combustion timeline, the result is also a higher pressure at the muzzle when the bullet exits. That means more blast, more flash for a given amount of flash suppressant, and more eardrum-rupturing noise.
Note that to stay strictly within the specifications of 38 Special we cannot take advantage of a higher MAP, but we most certainly must use slower burning, progressive powders -- otherwise the cartridge simply won't deliver adequate performance. This means that 38 Special will often have a higher muzzle pressure and produce more hearing-loss and tinitus-inducing noise than a cartridge loaded to a higher MAP like 357 or 9x19mm that uses a faster burning powder that drops the pressure lower before the bullet exits the muzzle while still delivering the same or greater velocities.
357 (9x33mm @ 35,000 psi) allows for a greater maximum average pressure than 38 Special (9x29mm at 17,500 or 20,000 psi), and it allows for larger volumes of slower-burning and more progressive rate powders than 9x19mm at 35,000 psi. The higher pressure and larger volume of slower powders make 357 velocities higher for any given barrel length. The greater MAP of 357 vs 38 is very significant even in the shortest barrels. Short barrels drastically reduce the benefit of slow, progressive powders. There is still a benefit to them even with short barrels, but it ranges from perhaps as little as 100 fps in a snubnose barrel to as much as 1000 fps in a long (rifle) barrel. This is why we see an LCR in 9x19mm producing velocities only a bit behind a 357 Magnum LCR, but once we get to a 4" barrel or longer, the 9x19 cannot produce anywhere near the velocities of a suitably-loaded 357 Magnum. Now whether the extra velocity is useful for anything is another topic. It certainly produces a flatter trajectory out to longer ranges for hunting, but as long as we're talking snubnosed revolvers, we're probably not hunting.
Both the 357 and 9mm's greater MAP versus 38 Special offer a decided advantage even in the shortest barrels. Not only do they offer sufficient acceleration to achieve the velocities where most bullets in this caliber are most effective, but they can also be loaded to do it with a higher initial peak pressure and a lower muzzle pressure. A lot can be said for the advantages and performance of 9mm for these reasons and many of the other reasons that make it the overwhelming most popular self-defense cartridge, but from a revolver like a 9mm LCR, you might want to use bullets with a crimp groove and crimp them to avoid crimp jump that can disable a revolver cylinder turning. Of course, that would almost certainly involve handloading.
