The "ultimate pressure" of BP is around 25,000 psi. That is, when it is packed into a strong container which will not burst when it is ignited.
I don't know what the "ultimate pressure" of smokeless is (probably partly depends on whether it is single- or double-based), but I do know that loads were developed for destruction-testing of the M1 Garand which ran up to 125,000 psi. This was the most that could be developed and measured with the rifle powders used in that cartridge, and with a bullet moving down the barrel to increase the volume as it burned.*
While both types of powder burn faster and faster as the pressure rises, BP is still limited to that 25,000 psi.
In both, burning rates versus pressure can be modified by such things as granule size, deterrent coatings, hollow granules, etc. However, smokeless rates can be varied over a wider range since it can be molded or extruded with different shapes, more than one hole may be formed while the powder is being processed, nitroglycerine can be added (which is "double-based" powder), and with other techniques.
As an example, the burning rate of "ball" powder can be increased by putting the round granules which are formed in its making through rolling presses, which increases the burning versus pressure rate because of the flattening.
As to the "foomf" of black powder versus the "hiss" of smokeless when burned in the open, I have long suspected that the individual powder grains of BP are lighter and less bound together internally, unlike smokeless. Thus the initial ignition of a loose pile of BP blows the other grains some distance apart, allowing the flame to reach a bunch of other grains all at once. And, maybe even more importantly, they are easier to ignite than smokeless.
Also on the other hand, smokeless grains are dense pieces of plastic which have internal cohesiveness and will not be broken apart or blown a distance apart from each other. So the flame front passes through the pile more slowly than BP.
Sort of like burning a stick versus burning a sheet of paper.
To be rigorously technical about it, BP is fluffier than smokeless.
That's my long term suspicion, anyhow.
With respect to "propellant" versus "explosive" for smokeless versus BP, igniting a closed can of smokeless results in the can finally popping open with the remainder of the smokeless hissing away, since the small amount of pressure required to pop open the can along its seams is released immediately.
With BP in a closed can, however, the can will be ripped open and flattened out and possibly thrown a dozen yards, since once the BP is ignited, it all burns up almost instantly, with a loud report.
These experiments were actually performed by General Hatcher, as described in the cited work below.
Terry, 230RN
* See
Hatcher's Notebook in the chapter titled "The strength of military rifles."
