Something else just came to mind, "sparked" by a comment we often hear regarding B.P being hydroscopic. It really isn't to any great degree. The fouling is. That said, I can say from experience that Pyrodex [ unfired] is hydroscopic . You need to seal the container well. The same is true of some of the other substitute powders. Not a good thing.
BP subs and hygroscopic properties.
When you look at what constitutes a powder grain in most of these BP subs you are looking at powder grains that were not produced as the result of any compacting or densification process. These grains are little more than loosely bonded large agglomerations of the pre-ground ingredients. The pre-ground ingredients are blended together in the desired proportions and then the mix is slowly "wetted" while being mixed or agitated. The dampened ingredient particles then simply stick together. They generally lack mechanical strength as is seen in black powder grains that came out of compacted press densified "press cakes". If you look at these bp sub grains under magnification you can see the large "pores" in the grains. Almost "sponge like" in appearance. Lots of room for moisture from the surrounding air.
When you look at patent disclosures for a lot of these subs they state that by not going into press densification and then "corning" their method of production is far safer than that used to produce dense compacted grains of black powder.
This thing about grains being little more than large agglomerations of the basic ingredients is one of the reasons most of the subs will not give as uniform a shot to shot consistency as black powder. A properly made black powder is something of a gold standard for shot to shot consistency in the gun. Go back and look at the claims once made by Ely where they claimed a shot to shot variation of no more than 5 fps with their higher priced bp cartridges. With BP grains you want good density, a highly polished grain and a certain degree of uniform grain size in a particular loading.
It is a bit more complex than what I have gone through but some of the subs are open violations of all that was holy in bp mfg. But then they are not catering to a technically astute customer base.
Bill K.