Black-powder cartridges tend to be straight or nearly straight cases with large capacities and large-diameter bullets. This poses quite a dilemma for smokeless powders.
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<br>On the one hand, the large bore bleeds pressure away fast, so it appears that a fast-burning [smokeless] powder would be the best choice in terms of the interior ballistics of the cartridge.
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<br>On the other hand, the powder charge should be voluminous enough to fill or very nearly fill the large case. A caseful -- or nearly a caseful -- of fast-burning smokeless powder would be a drastic, severe overload.
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<br>The relative quickness (erroneously called "burning rate," which is something else) of a smokeless powder varies with the pressures that powder is allowed or forced to produce when it burns inside a chambered cartridge. The larger the case's capacity and the powder charge, and the smaller its bore, the faster it burns.
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<br>Put another way -- as the burning of a smokeless powder builds pressure, that increasing pressure burns the powder faster. Low pressure, slow burning -- high pressure, fast burning -- higher pressure, faster burning -- same powder, different "burning rates" at different pressure levels.
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<br>Either way you load smokeless powder into a typical black-powder case, there's a problem -- a caseful of slower powder won't burn well, because the bullet begins to move too soon and moves too fast to allow the pressure to climb fast enough to burn the slower powder efficiently. IOW, the large bore doesn't allow the slower powder to develop enough pressure to develop its full potential. Its sound may seem like an explosive BANG! to the ears, but in terms of its interior and exterior ballistics, it's just a loud chemical belch.
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<br>A caseful of faster powder builds too much pressure too fast for the gun to stay together, and significantly less than a caseful leaves too much unfilled space in the case for the fast powder to burn consistently enough to make a good load.
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<br>And the black-powder guns are usually too soft to bear the forces and pressures that develop so fast with smokeless powders that are fast enough to work in large-capacity, large-bore cartridges.
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<br>Since no one is going to be satisfied with the very low velocities from a caseful of slow smokeless, the smokeless powder therefore almost certain to be used is a fast powder. A charge small enough to be safe doesn't fill much of the case, so its pressures are sure to be erratic (depending heavily on where the powder is when the primer flame ignites it -- back next to the primer vent? forward against the base of the bullet? in a shallow layer along the bottom side of the case?). A charge of fast smokeless large enough to produce a decent velocity when it ignites back against the web can be 'way too hot when it ignites up against the base of the bullet.
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<br>Inert fillers (kapok, poly fiber) and wads sometimes ameliorate the problem but don't solve it, and sometimes they even increase it enough to be dangerous in their own right.
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<br>A completely cylindrical case allows the bullet to be seated deep enough to reduce the net capacity enough to match the net powder space to the fast smokeless powder, but this seating usually makes the bullet completely disappear down into the case -- far from the throat and esthetically unacceptable to most shooters even if it works well.
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<br>A safe load of smokeless powder in a large-capacity, large-bore black-powder cartridge is a delicate balance at best. So even the "experts" who understand the intricacy of all this prefer to give only the easy warning ("Don't do it") rather than the involved explanation -- which they know that a large number if not the majority of handloaders won't bother with anyway.
