Black Powder Mythology
Can we rename this thread "Black Powder Mythology"?
In short: The reaction speeds (burn rates) *can be* slower for smokeless powders, but they can still produce higher pressures in small arms because they produce more gas for a given reaction rate. Smokeless reaction rates are easily tailored for specific applications, and for us, it is as easy as picking up different types off of the shelf to suit whatever we are loading. Reaction rates for black powder are less variable.
Barrel length: As long is there is sufficient pressure to overcome resistance, increasing barrel length will increase velocity, with BP and Smokeless.
Somewhat briefly:
Gunpowder (BP) and nitrocellulose-based propellants (Smokeless) are both considered to be "low" explosives when classified by response. "Low" means that they "deflagrate" instead of "detonate". "Detonation" is when the reaction rate exceeds the speed of sound in the material and creates a shockwave. "Deflagration" is subsonic combustion (burning).
For transportation purposes both BP and Smokeless can, under specific circumstances, be labeled as Class 4.1 "Flammable Solids". See Title 49 CFR, and keep your migraine medicine handy. (
I'm not a legal expert or any other type of expert, but I can read about as well as anyone. There is a Haz Mat table in Part 172, and § 173.170 and § 173.171 may be of interest. )
Strictly speaking, the reaction speed *can be* greater in NC than BP. However, in small arms applications, the reaction speed of NC is typically modified (retarded) to suit particular uses.
Both types of propellants work the same way: They generate large volumes of gas very quickly by deflagration (burning very quickly). In an enclosed space such as you find in small arms, this creates usable pressure for lobbing projectiles. NC propellants are much more efficient in that the vast majority of their reaction products are gaseous. BP is much less efficient, as much of the reaction products are solids (about half, noticeable as fouling).
But don't take my word for it. There is plenty of info on the net, and I think we learn and understand more by researching than by reading someone else's answers or opinions.